I setup a WebDAV server locally with my network have 600M/s bandwidth, tested single file download speed for both free WebDAV client WinSCP 5.19.5 and FTPRush 3.4.6.
Here is my result.
With TLS encryption (ECDSA-256 Algorithm)
WinSCP: ~32M/s (only around 40% compared to no encryption)
FTPRush: ~68M/s (around 95% compared to no encryption)
From the result you can see that under TLS encryption environment, FTPRush can still keep around the same speed compared to no encryption, but WinSCP only can get around 40% speed compared to no encryption. Could you please figure out and fix the speed of TLS encryption of WebDAV connection? Appreciated for that. You can easy to setup the environment, FTPRush can be download from https://www.ftprush.com, WebDAV server software can be download from GitHub: https://github.com/hacdias/webdav, below are the commands I used to generate key and cert by OpenSSL, for your reference.
openssl ecparam -genkey -name prime256v1 -out key.pem openssl req -new -x509 -days 3650 -key key.pem -out cert.pem