New GPU, lower FPS

Exactly—there’s really no need to swap out the thermal paste. In fact, it actually needs a little bit of time to "cure" properly to reach its peak performance levels...

Try completely powering down the PC—like, pull the plug entirely—and then go ahead and reset the CMOS. You can just pop the battery out, give it about a minute to settle, and then put everything back together. Once you're back up, just jump into the BIOS to fix the system time and those other little settings, and then see if that does the trick.

The thermal paste on my CPU is seven years old and still performs flawlessly. Why bother stressing the processor, the cooler, or the motherboard? In my experience, the best paste is always one containing high-quality silver.

I have already stated on this forum multiple times: when you install new hardware like RAM or a GPU, you should reset the CMOS. Yet, some people refuse to believe it, and others simply refuse to read. If they want to waste their mental energy and fry their nerves, let them.

Furthermore, always check the power supply first. I have mentioned how to verify its stability several times already.
The PSU is the only component that isn't guaranteed to last. Unless you overspec it by about 30%, it won't hold up. With that kind of headroom, it can easily run for five or even ten years.

There is more to say, but I am done. I have no desire to continue when incompetent people keep arguing against me.
They can all go to hell.

What is your problem, man? 😕