I fear that I may have damaged my CPU.
It still works, but I have noticed a huge performance decrease.
A game I play called Planetside 2 has a handy feature that shows my FPS and my current bottleneck. 1 week ago, the bottleneck used to be purely the GPU, now it always the CPU. It runs at less than half the frame rate. So do all my other games that I have tested.
2 months ago, I overclocked my i5 3570k to 4.2 ghz at temperatures that were around 60-70 Celsius. Now it is 10 degrees higher, at 70-80 degrees Celsius. I reapplied thermal paste and replaced my heatsink with an identical one. No change.
I recently checked the Intel website for the Tcase (max temp) of the 3570k and it said that it should be ~67 degrees. Obviously I didn't know this at the time, so it has been overheating.
My question is, have I damaged my CPU? Could I have impaired performance? Any way to fix this? Or could this be something else?
If the temperatures were 60-70 prior to overclocking and they are now 70-80 after overclocking then those temperatures are fine. Just go buy yourself a new heatsink to correct the new temperature change. I would recommend the Hyper 212 EVO if on a budget.
Edit: If you also tinkered with the CPU Voltage then you should try and see if you can lower it even more while still being a stable overclock. Voltage generally increases temperatures.
To answer your question, you probably have NOT irreparably damaged your CPU. If you hit ~100 then yes, you would have. If you run it at ~90 for like a year you also probably would damage it. Right now you can most likely just lower your temperatures with a better cooler or paste etc. and it will be fine.
Which other games are you playing? If you have a game which is CPU heavy but light on graphics (turn-based strategy like CIV) you could try playing that and if the issue really is your CPU, game performance will be very noticeably slow.