Found that Rhythmbox 3.4.2 sync is slow and the CPU load is about 40% on its process. Also it deletes some files from SD card before sync then puts it back again. Looks like it re-encodes tracks to fit some constraint that significantly slows down sync.
How to force skipping encode to direct copiyng files? It is proven that player supports directly copied files.
Xubuntu 18.04.3 LTS
Here is .is_audio_player
audio_folders=/ folder_depth=0 output_formats=audio/mpeg,audio/mp3
And sync settings