We have a bug report - T88294 - that describes very bad performance while appending under extremely slow (and very high latency) conditions. Specifically the files are hosted on a network drive shared on a WLAN (over the internet) with clients using home WIFI. Under the user’s conditions they are describing hangs of minutes to access a 20MB blend file for appending. I have been unable to recreate such bad conditions myself and am hoping someone else can do so.
If anyone can recreate such bad behavior, then I’d love them to test the following patch which tells the OS that it would be a good idea to preload the file (or parts of it) in the background rather than waiting for the user to browse through it. The patch description describes what it really does in greater detail.