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Computer slows down when connected to external monitors

I am working on a MacBook Pro that has dual GPUS (Intel Iris & AMD Radeon)

The user uses 2 external monitors.

His computer starts slowing down, the kernel usage gets to 87% and oddly bounces as high as 240 %. The laptop is a bit warmer than usual but no thermal problems. The user reports that when this problem started that it was about 1 hour or so before the computer slows down; it is now starting to slow down in about 5 minutes.

The user has reset the PRAM and also SMC reset but no change.

Is this a hardware failure? Thanks

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Having experienced this issue myself using a MBP with 2 x LG LED 24EN43 monitors; I read these comments with interest. Although not mentioned, I just unplugged the monitors and went to System Preferences/Displays/Colour then hit the Calibrate button. That was a perfect fix. Everything works like brand new! Hope this helps you too.

Another useful tip from...


Sorry mate, I don't see how color calibration (which is just a software profile) would affect the CPU performance, especially when I (I don't know about the others) am experiencing this issue even on boot camp in Windows! Regardless, I tried your suggestion, and it made absolutely no difference; after 30 seconds the laptop was throttling again.

He means that he 'disconnected' permanently monitors and calibrated his laptop instead :) Never connected them back ;)

In my particular case, I actually found out after lots of frustration that my laptop power cord has an issue and while it was connected, I had to bend the cord this way or that in order to have it actually charging. When I took the power cord out of the equation, everything returned to normal. I'm running 3 external monitors...one through an external USB-based video card that I'm using 1 DVI port, one through the existing laptop HDMI port and one through the existing laptop VGA port. Up until my issues this afternoon, all 4 screens worked flawlessly...then all of the sudden it was like my computer was bogged down with too many tasks...just moving the mouse across the screen was a multi-minute chore...so, if anybody else is troubleshooting, as strange as it sounds...check that your power cord is working properly.

I was able to fix the problem by matching the display resolution for both monitors. When the laptop monitor and the usb monitor were the same resolution there was no slowing.

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Seems like the gpu just cant handle the extra horsepower needed to run the monitors. Try it with no monitors and see how long it lasts without them.

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This set up has been running for about a year without problem. This just started recently.

Try that. There might be issues with the GPU as it seems to be with almost EVERY SINGLE macbook ever made

It does work better using just the MBP display. Is this a hardware failure of the GPU or is it something else?

Could be hardware failure of the GPU. Like i said try it with no monitors at all and see how long it lasts. Does the same thing happen with no monitors as fast as it did before?

This model has 2 graphics adapters. It looks like the Intel iris is used for the display and the Radeon is used for external. It does run longer with out the external monitors connected.

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The problem seems to go away if you decide to turn off “Displays have separate Spaces” from System Preferences —-> Mission Control. Of course, this means you don’t really have two separate split screens. However, it seems this is the reason why the WindowServer goes nuts in terms of CPU utilization. Seems like an OS-level issue, not really a hardware problem.

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Thanks! This solved my problem.

I'm using:

macOS Mojave

Macbook pro 15" Mid 2015

1 external display

Solved the problem of my friend as well. This is the right answer to the problem. Very helpful.

Same here!!! Solved the problem.

For my Late 2013 15" MBP this only provided temporary relief. What did work was to stop running Macs Fan Control, found here: https://superuser.com/a/1295928/1139958

Update to that: the Macs Fan Control people tell me it was because of my wrong configuration of the fan speeds

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I would start off by opening the system and cleaning the dust out of the fans. Make sure both working.

I would also download this app: TG Pro even using the free version might clue you in on whats happening with the heat transfer from the CPU and/or GPU. The full version offers more for less than $20 US its worth it!

Look to see if the heatsink temp is following the GPU/CPU's temp. If not the heatsink has likely lost its coolant so the CPU or GPU is not able to shed its heat so the system is throttling down.

If the values are the same then you'll need to look deeper into the system as well as see if the drive is too full. SSD's need a bit of wiggle room for wear-evening. In addition, if the system is only a 8 GB model the system maybe using the SSD for virtual RAM and depending on the app cache or paging.

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I have the exact same problem. However, although this answer has been marked as being the solution, I don't really get what the actual solution is? Because of this phenomenon, I had my logic board replaced 4 times, and also the heat paste has been changed. But still the problem remains, even if only one external monitor is attached to the MacBook.

The answer is more of a process than > Bang replace this! Type of answer. So did you follow the process? If you did post your outputs so we can see it to get to the next step.

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Have you checked if the display settings are using the external monitors? If it is thats probably it. I changed it back to display and it stopped lagging

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1 Comment:

Hey, could you elaborate more on this? This seems to be happening to me.

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I have had a very similar problem with MacBook Pro and Samsung external display. So after an hour on support to Apple and Samsung doing PRAM resets etc and both blaming each other,

Apple mentioned that third party software could be causing it. I was running Flux (which is a colour temperature app. Like nightshift). Removed that and it worked instantly. Check any third party apps that might be running in the background and then reintroduce them.

Good luck everyone!

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1 Comment:

this seems the exact problem i had and your solution works. Thank you so much.

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Can you adjust screen resolution se if that helps?

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I don't think it's relevant. The problem seems to be that when MacBooks are connected to an external screen via thunderbolt, there is not enough power going through to the processor, hence forcing it to slow down.

I'm having the same issue with just a HDMI cable plugged directly into the MBP (early 2013), so not related to Thunderbolt on my system.

It even happens with a VGA adapter.

Well ... it actually happens when the discrete GPU is in use, and it is equally responsible for HDMI and any TB usage ... soooo ... normal that it misbehaves the same ;)

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For me the problem disappeared when I connected my MBP to my monitors HDMI ports instead of the TB ports.

I have MBP 15” 2017, Lenovo ThinkVision 4K monitor and an old Samsung HD monitor.

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I have similar problem.

I connected an external monitor (4k) with DisplayPort to MBP 15’’ mid 15 (m370x).

And I see that CPU speed is slow down.

I use Intel Power Gadget for monitoring.

The Cpu temperature never gets higher than 60 degrees, but Cpu speed easily can be stuck on 0.8Ghz for 20minutes or more. For the short period, it can work in the 1.5-2.5 range, but after that stuck again on 0.8Ghz. I thought I paid for 2.5Ghz + turbobust.

Now It gives me «1500 Multi-Core score» in geekbench and it’s impossible to work on this machine. Without external monitor 14000 scores.

It looks like the real bug.

What is going on? How to fix it?

PS: I also tried HDMI port and low resolution. It gives the same result.

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I never found a solution to that problem. But I noticed that it gets a bit better if I disable the internal monitor by closing the MacBook's lid. It seems as if the MBP 2015 isn't built to run two monitors at once in a somewhat warm environment. Which is ridiculous, if you ask me.

All these problems look weird. 2015 was built for 2 monitors (it has 2gpu, HDMI, and thunderbolt). It looks like a baby problem. Unfortunately, apple doing their laptop worse and worse. I had 2011 with every year burning GPU, i had iMac 5k with phantoms on the screen, 2015 with oleophobic problems.

But slow down CPU with the external display, it's really too much. Nice accessories, but this is not "pro" and this is not compatible for work.

PS: The most ridiculous, that I message on ifixit, and i cannot message on apple, because they don't have support. No chat, no forum, nothing. They disabled chat, their "discussions" doesn't work. No support - no problem.

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I was experiencing extremely HIGH Kernel_task CPU usage whenever I was trying to stream on Youtube. I have a 2017 MBP 15 inch hooked up to 2 monitors via USB. One is connected directly to the MBP and the other one is connected as a pass-through a CalDigit TS3+ dock and with Airplay on the Apple TV. And what caused the problem for my system is simply Apple’s HiDPI scaling. If I set any type of scaling within system preferences Kernel_task cpu usage will shoot up to 500% within minutes. And any type of streaming especially Youtube will stutter having trouble loading videos smoothly. Took a lot of time digging around the internet until ironically I stumble upon a Youtube video showing similar issues referring that the culprit is HiDPI scaling. When I reset the main display’s resolution to default the MPB is back to normal and works just fine. But that’s not an ideal resolution to work with (1650x1050). So I use SwitchResX to adjust the resolution. But there’s a catch… Again, cannot select any settings that is HiDPI otherwise the CPU will start to throttle again. It seems like Apple did not make HiDPI mode to work coherently when connected to any external monitors. Results may differ if it is connected to an Apple Display.

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Setting my fan control back to automatic worked for me. Ty

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For me up until now this was the solution for 500+% kernel CPU !!!
kernel_task takes up 500% of my CPU when plugging in external monitor

I have also disabled Seperate Spaces for both monitors but not sure if this is doing anything…

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