One possible reason for slowness I've read about is that Windows Defender scans relevant files every time a program is started. This method of working might always cause a delay. If a different AV program works the same way, then perhaps this would also cause a delay. Other AV programs may not work the same. Having scanned the files once, they are known to be safe and hence it wouldn't be necessary to scan them again.

Whether a program opens speedily under Windows Defender might depend on the files (or the number of files) being scanned. All just a theory though. Other security programs I've tried that don't cause a lag are:

Avira Pro.
HitmanPro.Alert3.
Malwarebytes Pro (running realtime).
Bitdefender Internet Security.

All these being run with Windows Defender disabled. XDP opens much more quickly under these conditions.