I recieved an answer from Veeam suport regarding this issue (case 00203915)
Support confirmed that they were aware of the problem, and provided a workaround: OtherOS FLR with FTP
Although this is a bit more timeconsuming to set up, the restore/copy speed was much faster, and totaly acceptable.
Complete answer from Support:
We are aware of the low performance while restoring multiple small-sized files via Windows FLR. This behaviour will definitely be improved in the future versions of Backup&Replication.
As a workaround here I suggest using OtherOS FLR with the FTP enabled. I've just tested it in our test environment and was able to get decent speed and restore some files with Norwegian symbols in their names. Basically, you just need to connect to the FLR aplliance's IP through FTP protocol using Windows Explorer or any FTP client you like and copy the files to original or any other server.
Could you please try this method and tell me if it works for you?