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I had something similar when my drive started to fail.
At first, it was annoying, because the cursor froze all the time, just like yours, then programs started to do the same, then they started to crash without reason, and in the end, even my unbreakable OS (Fedora Atomic) broke randomy and incoherently.
What did I learn? Don't cheap out on drives, and keep enough backups.
Yeah, I'd be checking the smart stats on my drives and doing a full data backup if I didn't already have one.
The SMART didn't help. It showed full health and no errors.
While that's encouraging, I would still make a full backup of my data and have a plan to replace the drive while I investigated further.
Chiming in to say that I've also experienced this on systems with an unresponsive NFS mount, although in that case it hangs until the connection is restored or the network operation times out.
Years ago I had a similar problem that was resolved by changing the method of I/O scheduling.
https://www.techrepublic.com/article/how-to-change-the-linux-io-scheduler-to-fit-your-needs/
I'm not sure which one I used or if this is still relevant with modern Linux but it's something to look into that might help.
Switch to zen kernel, use preempt=full kernel parameter.
Happened to me as well, freezing up the whole system, especially with many/large files. None of the suggestions I found online actually worked. Happened on USB drives as well.
Then my whole HD gave up completely. Replaced it and all problems are gone. USB drives as well: turns out Linux at least tries to read it so you can attempt to save some data, while Windows just says it's unreadable.
It might be interesting to determine whether the freezes are limited to Plasma or are happening within the kernel.
Thank you all for your ideas, I managed to solve the problem. It was somewhat hardware related, but not really.
I was using BTRFS, which creates a lot of write amplification and aged SSDs don't handle that well. According to a study from 2017, btrfs can cause up to 32x write amplification, absolutely hammering its performance: https://ar5iv.labs.arxiv.org/html/1707.08514
I converted my system to RAID0 using EXT4 and the stutters and freezes are gone.