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.

what are you using to copy? what filesystem is the usb stick using?

I'm using LMDE 7 and I cannot reproduce this.

My setup is a no name USB 3.0 32g usb stick formatted exFAT. I tested using cp to copy a 170k file to the usb stick and it took less than half a second. I ran it with time to make sure. When I ejected the drive it took maybe 1 second more before it was ready to remove.

ETA: I just tested a 1G file and it took 45 seconds to copy and run sync.

This has been my goto fix on a number of problematic machines:

Add these lines to the end of /etc/sysctl.d/99-sysctl.conf

vm.dirty_bytes=50331648

vm.dirty_background_bytes=16777216