Hey @Space.yg, welcome to the Nextcloud community and thanks for supporting Nextcloud snap 
Sorry to hear you are having issues with your Nextcloud snap installation. If you’ve come this far, you’re on the right track! There is a good chance someone in the community has had a similar issue and knows how to resolve it.
Rest assured, the snap does not throttle anything. So the issue you are describing may be an issue with that client. You seem certain the bottleneck is not a network issue? You did enable HTTPS compression right?
Now I personally have in excess of 6K small files (mainly markdown documents and plenty bash scripts) and although I’ve never used the Nextcloud desktop client, because sync is not backup and sync is not copy, I can agree with you that transferring many small files feels slow (kbps) although I have plenty bandwidth (down 100 up 30).
This is why I prefer a scripted solution using rclone which also automatically backs up my docs to NAS.