Computer lags with GPU0 at 100% and GPU1 at 0%

It seems that the second GPU wasn't configured correctly, so it is detected, but doesn't get used by your applications. I found a solution that you could try: use Nvidia GPU instead of integrated intel card

If Firefox uses too many resources, check your extensions or reinstall Firefox.

To the resourceful “System” process: do a right click on it and check the location of the binary. Bitcoin miner often use the GPU, so it might be one of these. Also, you can use Microsoft's Malicious Software Remover Tool to check your PC.

At the end, if nothing works, check your drivers. I'm recommending IOBit Driver Booster for this. If all your drivers work, backup your data and reset your PC. This helps in most Software related problems.