Current setup
So I am now backing up my personal data from my internal ext4 HDD to an external ext4 HDD via USB3, using FreeFileSync. It is a brand new PC, all components pretty fast and fancy. Linux Mint Cinnamon 20.2.
Problem
When I first start a large backup it has speeds of over 150 MB/s. Great! Even with a huge backup of 200GB, FFS shows it will only take 20 mins or so. On my old PC (USB2 only) I'd have been waiting hours.
However, after a short period of time (from 30 seconds to ten minutes) the speed starts dropping massively. Doesn't matter if it is on a big file like an iso or loads of tiny ones. It ends up dropping below 1MBs, sometimes to 0KBs, but then it gets faster again, then slower, then faster. Kind of like a pulse, but the higher speeds are a bit lower each time, and it never goes back up to the fast speeds - max is c, 30MBs, but often at a crawl. It's as if the data transfer speed is being throttled.
During this time I have a system monitor running. My PC has 32GB RAM, but even during the backup only 2GB is being used by the OS. The CPU goes from c.1% usage up to c.20-30%.
If I reboot and try it again, I get the 150MB/s speed again, but by the time it's done 10GB or so it starts dropping again.
So my question is, what causes the slowdown? A cacheing issue with USB3? But in that case, why does it never catch up again, especially when I have so much memory? Instead of the backup taking half an hour, I had to stagger it over three days and multiple reboots to get it finished. Since it's obvious my PC can do 150MB/s, that's pretty frustrating. Since FreeFileSync estimates the end time based on current transfer speed, I watch with dismay as the "20 minutes" estimate grows and grows until it is saying "22 days"!
The issue of USB3 data backups to a HDD being fast, then suddenly dropping, seems to be quite common. One post (which I now can't find) said it might be related to a certain part of USB3 (it had an acronym) which can be turned off, and then people get faster transfer speeds because it no longer throttles them. I tried the procedure in the post at viewtopic.php?p=1636401#p1636401 but it made no difference (so I undid it again).
Obviously this is not a critical problem, just inconvenient and a bit irritating!
(like mine 
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