WebDAV upload is slower than download in intranet

Intranet
Server: Apache httpd 2.4.41, default config, Win10, 1.0Gbits LAN
Client: WinSCP 5.15.5, default config, win10, 5g wifi at 866Mbps
Protocol: WebDAV, HTTPS
Download file from server: 25MB/s
Upload file to server: 12MB/s
Upload is much slower than download, why?

PS:
Same server, network, protocol
Client: ES file manager, Android phone P20Pro, 5g wifi at 866Mbps
Download file from server: 60MB/s
Upload file to server: 10MB/s

So do I understand correctly that you get slower uploads in other WebDAV clients too?

Yes.
No upload rate limit has been set in Apache.
As I know, WinSCP supports multi-thread download.
Dose WinSCP support multi-thread upload?
Does WebDAV protocol support multi-thread upload?

What do you mean by "multi-thread download/upload"? WinSCP supports downloading and uploading multiple files in parallel from and to a server (using any protocol).

There is an option "Multiple connections for single transfer". Does it mean download/upload one file using several connections?

No. That is not supported:
Issue 513 – Transfer single file using multiple connections