Weird behavior/ sluggish after handling large files

I've been experiencing a weird problem on my PC for a long time and it's driving me nuts.

When I first boot it up it's fine, and as long as I don't handle any large files (like tens of gigabytes or larger), it's fine for days. But if I start copying large files from drive to drive, or large groups of files, then it soon starts acting very sluggish, especially when opening new windows or anything that accesses files.

After I've copied large files, if I open my computer, it takes like 30 seconds for the C drive to appear with the proper "space left" bar underneath - the green "working" bar at the top of the window slowly moves along as it's looking at the drive. It instantaneous when I first boot my PC and stays that way unless I'm working with large files. Same for right-clicking files - if I right-click a file to do something with it, it takes a while for Windows to look at it and give me the right-click menu. Again, it's fine until I start handling large files.

Also, only the C drive does this. Any other attached drives pop right up when I open my computer, though right-clicking still takes just as long even on other drives.

The only way to correct it is to reboot, and then it's fine for days, unless I start handling large files again.

What could be causing this? I have 16GB of memory. I do use Fastcopy for all my copying. Could that be causing the issue? Is it a memory issue?

I doubt it's indexing. It does not improve until I reboot the PC, so it's not like it only happens briefly after copying.

I don't use encryption.

I've done a chkdsk C: /scan in recent weeks and it returned no problems. This has been happening for a long time. Maybe since I built the PC 2 years ago. I can't remember when it started.

I'm talking folders of large archived files, like 50GB of multiple rar/zip files. This is happening even when copying from one internal drive to another in my PC.

Files copy fast, and verify fast, even after the sluggishness starts up. Copying isn't affected. Only initially accessing the folders/files/ C drive.

Yes, all copying completes and verifies using fastcopy, and even after I notice the sluggishness at some point after I've finished a copying operation, additional copying is still fine. Just interacting with files, like right-clicking, is slow.

I'll have to try copying with just Windows file explorer later this afternoon. I'll report back.

For an example, I rebooted my PC a while ago, and just finished copying 44GB of files (a folder with 25 2GB rar files inside). It finished, and so far my PC is normal - no sluggishness. But at some point if I keep copying more large folders, it will start acting up. It's really weird.

Well I just rar'd up a 20GB file to another drive using winrar, and now the computer is acting up like I described. So it was fine before rar'ing this file, but now it's sluggish again. So it's not just copying files that leads to this.

I tend to copy files and archive/rar around the same time, so usually when I do one I do the other after afterwards. So maybe it's the rar'ing up of files and not copying? But why should that be happening?