Sync is extremely low whith large amount of files (desktop client)

This topic is similar to this one, but this one is 4 years old and does not help me.

I have a ~500MB directory, with approximately 20,000 files. The syncing of this folder by Nextcloud desktop client is extremely slow (several days), and I wasn’t able to find the files mentionned in the answer in the previous thread.

I am on Macbook air M3, and I installed the client with the official website way (.pkg file, I now have a Nextcloud.app application)

Are the files roughly similarly sized? As in are these all relatively tiny files? Can you also post your server config (occ config:list system)?

No, I have a lot of different sized files, but never more than a few mega bytes. What takes time is the number of files, not their size.
I dont think I can acces a server config, the nextcloud server is hold by my laboratory.