Really slow file transfer from Ubuntu 18.04 to USB 2.0 pendrives

It's probably crappy USB drive. Cheap USB drives have a write speed of <15MB/s, some as low as 1-2MB/s. Want faster? Buy an more expensive one.

10 minutes for 2GB works out to approx. 3.3MB/s, which is reasonable for a cheap drive.

The reason it show a quick progress is memory caching. The file is only written at the end of the copy process, which tricks the UI into believing that copying is further along than it actually is.

(As a sidenote: I've even seen drives advertised as USB 3.0... and 5MB/s write speed!)

try to copy files from the command line to know if nautilus faults

cp /source-path/source-file /dest-part/dest-file