Windows 10 apparently has much improved audio processing, so that would be why you are noticing improvements in audio.
I have no idea if the video card is a problem, but I would certainly check on the manufacturer's site to see if there was a later Windows 10 driver for it. As we all know, sometimes Microsoft doesn't provide the latest driver, even in major upgrades!
If you want to check the GPU load,
GPU-Z has a "GPU Load" measurement on its Sensor tab. Of course the GPU could be offloading work to the CPU, but I always check the load on both when trying to track down what is going on in Windows.
If it definitely is MC that is using the CPU, and it starts when the PC is idle, then there must be some background process that is starting up. It could be audio analysis.
I would guess the only way to know is to have "Help/System Info" page up, with the mouse over the "Refresh" button, and wait for the problem to occur, then click refresh. The report gives you a snapshot at that moment, so if some background tool is running it should show it.
Of course, there have been some reports of ESET causing these sorts of issues, so you probably need to look at what it is doing. You have only excluded MC program directories, and not all your media files. Also, you have StrongVPN running, which could be doing something. You probably need to look carefully at Windows Resource Monitor to see what is using CPU cycles, using the network, and so on, even when idle.