The real-time scanning mode of Windows Defender completely destroyed the performance of Cygwin Setup when accessing a mirror stored on my NAS, to take one example. I'm not talking about a few minutes extra; it issued loads of network requests for every signature verification Setup tried to do, the process was still non-responsive after several hours. Turned off realtime scanning, and it immediately finished.

Realtime monitoring has the biggest risk of performance degradation.

Is there any way to turn off Windows Defender without installing anything else?

Yeah, through the new Windows control panel ("Settings" -> "Update & Security" -> "Windows Defender"). Windows will hound you about that (and there is no way I know to turn off the nag).

I've heard if you disable the service directly, then the "Windows Defender reports that the service is turned off" message stops happening.

  sc.exe config "WinDefend" start= disabled
  sc.exe stop "WinDefend"

No good I'm afraid, gave me:

    [SC] OpenService FAILED 5:

    Access is denied.

(this is from an administrator command prompt)

For the record: you can turn it off using the group policy editor.

I thought it didn't just nag but turned itself back on after a while?

As far as I know, only if you fall for the dark pattern nag. I haven't run in to this, either because I haven't had it disabled for long enough or because Microsoft buckled and removed it.