Your comments

I have the same storage strategy as you. SSD stores html, config, db.. and HDD stores userfiles. However, you can try comparing it to SFTP.

On my server, SFTP loads faster with almost no latency, while filerun via webdav loads a large number of files will be frozen for  a few seconds

the thumbnails was stored at .filerun.thumbnails which was a hidden folder 

Yes,just loading files list is slow.  both on web page  and  webdav  client.

When I click on a folder on a web page, the screen goes blank, and it takes five or six seconds for the file to load.

I'm sure it's not because of the network, because it's over a gigabit LAN, and there's no problem with opening folders with fewer files