Why does my FPS drop when copying files?

Can computers not multitask anymore or what?
I have a 12900k, 16gb RAM, RTX 3080, multiple NVME's, etc.

I'm copying files from one (SATA) platter drive to another. The game and Windows 11 are on an nvme SSD.

FPS drops by 20-40 while copying files in the background. CPU usage is very low, RAM usage is way below 16gb.

Is this just windows being lackluster or what is going on? Is losing this much performance really expected when simply copying files in the background?

Over on Microsoft Community site, there are reports of various things that have fixed FPS drops when background tasks were running that go all the way back to early versions of Windows 10. One that got a lot of responses was changing wallpaper settings to one static wallpaper instead of shuffle because the images were still changing in the background. Others reported it happening if youtube music was playing or Netflix was streaming or certain other programs were running.

If you haven't already, do a web search for "frame rates per second FPS drop while background process running" and another for "fix game stuttering with fps drops" to see if any of the fixes on the various game boards and tech sites work for you. There are too many for me to post here. It may be nothing more than your system is not configured to optimum performance.

Copying files does not occur directly form disk to disk - your system will be in use, even if very little, and that can easily affect your gaming as RAM is being used and CPU is being used, both of which are also being used by the game. How much is affected will depend upon lots of factors, including hardware, the game, your operating system setup, and so much more. Not ot mention, anti-malware software will usually be scanning files upon creation (when you copy to a new location) and this will use even more resources that may impact the game.

Best case scenario - if it makes gaming difficult, then you're probably better off choosing to do only one of those things at a time.

If you're gaming online, then online activities will affect things in more ways, too, because online gaming and other online activities will naturally fight for network bandwidth. But in your case, copying files locally won't affect the network bandwidth (unles you're copying to another computer, in which case it will).

Did see some versions back with the slower computing where it was best to have a Temp folder and a larger paging file/swapfile to assist in the copying from one drive to another, gave caching more room to work.