Strange things hardware related

[Kirika-]

Likely what has happened:

• Before CPU upgrade, the CPU was the system bottle neck, meaning the GPU was waiting (not working) for the CPU to catch up, which results in lower GPU utilization and lower GPU temperatures over all.
• After CPU upgrade, the system OS has to do a lot of internal reworking, which results in the system not actually reaching peak performance for a few days.... the same thing can happen after a major OS update, as in it take sometimes a few days for the OS to "settle in". So, during that time, your overall performance wasn't reaching its optimal rate, so everything GPU utilization and temperature wise looked about the same as before CPU upgrade.
• Then, once Windows finally finished tweaking itself for the new CPU, the GPU was now allowed to run at a much higher performance, but due to the higher performance, it's temperatures went up. Depending on the specific graphics card's BIOS settings for TDP, fan curves, and voltage/frequency values, the higher temperatures can result in the card "throttling down". The result is that you see higher temperatures and lower utilization.

Try using some GPU tweaking software for your specific card, or use EVGA's Precision X1 or MSI Afterburner to "get into" the card's BIOS settings to see where they are and make any tweaks that can help.

It can also be an issue that now the graphics card can run at a higher performance, you are hitting a limit of the cooling system for it and/or the PC case. As you have said the fans are working, I would then double check the case ventilation and maybe disassemble the graphics card to clean and re-paste the heat sink to GPU die contact and check/replace any thermopads for the VRMs and memory chips. Check YouTube for "Video Card Re-pasting" for How-To's on the process.
You can also try running the PC with the case open to see if the GPU temperatures and performance changes.