Is this only the CPU's cooler problem?

So my ibuypower element 9260 is a few years old and its really fast, but after around a week of use it gets really slow. After waiting a few weeks the fast performance is back. I was a little concerned about this so i just stopped using it but i decided that i wanted to revive it. I checked my CPU temp because the liquid coolers became really loud and sounded like something was about to pop inside my pc, the temp got to 90 C only after 3 minutes of it being idle (i checked a screen you can see if you spam the delete key after powering it on). I ordered the Thermalright Assassin X 120 refined cooler to replace the liquid cooler with (and new thermal paste). So i just wanna know if the liquid cooler is the only thing that needs replacing, i know the proper replacement steps but im still worried if i just wasted money. But then again, the pc still ran fast and the download speeds were really fast it just became slower overtime but became fast again after letting it rest for a few weeks.