New battery causing phone to be slow under 10%

I have the same problem as well. Did you find a way to fix it?

Same thing happens to me but when I hit 6%

Replace it again, if the problem persists, feel free to tag me in a reply that you have tried the part, because we could possibly be looking at something super fun :)

Add another of this issue, I only noticed it recently, because I rarely let my battery get under 50%, but somewhere between 10-20% this slow down is kicking in for me with an ifixit supplied battery, not particularly happy.. especially as a month later apple did the cheap battery replacement programme.

When you go below 20% and notice the lag can you try running Lirum Device Info Lite and check if it reports your device as using 100% CPU? That is what mine does but not sure why yet. Mine is an iPhone 6S, iOS 12.1, battery from ebay with apple logo covered with marker pen.

i had the same problem, i resolved it by doing a back up and restoring the backup.

i suppose it can work by updating the ios too

What probably happened is that the connector that you connect the battery to on the logic board is damaged. Either you can replace the logic board, which I would say isn’t worth it, or you can go to a logic board repair shop (there are a lot of them on eBay).

I believe that there is a bug (or a feature!) in the iOS kernel that when the battery level is low, it attempts to send commands to the battery which fail on non-genuine batteries, causing the commands to be sent repeatedly, causing the kernel to use 100% CPU and lock the phone’s UI up, and in my experience if I do manage to get a call out the audio is choppy. My solution was to find a genuine used battery from a reputable seller on eBay that deals in iPhone recycling, unfortunately with this particular seller it is pot luck to find one with a good health value.

I will suggest you to change this one and try new one. I hope this will work. All the best!