It really is surprisingly bad, isn't it? I recently was involved in a continuous deployment system targeting Windows. We simply couldn't get WinRM to reliably upload fast and ended up installing SSH everywhere.
Having worked on that code (in Packer) - the WinRM protocol is completely unsuitable for use as an SCP replacement. It's amazing it works at all even for small files! The actual "winrmcp" implementation is here https://github.com/packer-community/winrmcp if you're interested in the inner workings.
OK, but that code (cp.go) uploads base64 chunks of 8K and echo's them to a file and decodes when done. That surely isn't the real WinRM file copy implementation right?
Well... I just went looking for how it's implemented and holy shit I don't think they considered this case. Top results say "use a file share". Wow. I feel less inadequate.
Yep definitely the worst thing ever. Not just that though; everything. I've lost so much hair in the last few months over trying to get a CD environment up on windows.