I’ve had this issue since the first day I started using linux about 15 years ago. Always the same issue, transfers start fine and slows down to a crawl, only on USB sticks.
The USB sticks are fine, I can make bootable sticks in five minutes but if I copy a file of similar size to them is half an hour and then five to ten more minutes waiting to be able to safely remove the drive.
There are dozens of forum posts, some blaming the drives, others the GUI, dirty buffers, outdated kernels, fastboot for some reason. Others marked as solved without a solution because it magically solved itself.
Do you think you may be using USB sticks with a small SLC cache? If this is the case I would expect the transfer to be fast until the cache fills up then it would slow down to the speed of the rest of the NAND for remainder of the transfer. You wouldn’t experience the slow down unless you were transferring files larger than the cache. This could explain why the issue has followed you across so many computers.
More affordable USB sticks may have cache sizes of 1 - 2 GB. I’ve seen high performance models available with 64GB caches.
If this is the cause of your issue, using a USB stick with a larger cache would be a solution.