Windows Defender slowdowns are well tested. All operations on many files (copying, moving, installing, etc.) will be slower with WD as compared to some good AVs. Windows Defender has poor optimization for already checked files. WD has also a slow and heavy full scan.
So, the test results will be dependent on how many files are involved and on the speed of the disk. Also a poor Internet connection can probably have some impact, because testing many files require transferring file metadata (and files sometimes) to the cloud.
On my computer (SSD + 8GB RAM) I can see the slowdowns when Windows Updates are performed in the system and for updates of Microsoft Store Apps.
Normally, WD should not have a visible impact on the performance, when doing daily tasks like: web browsing, reading/editing documents, playing media files, etc. If not, then there is some kind of the software/hardware incompatibility, and the user has to choose another AV.
Updating WD signatures in this way, would be advantageous only when working offline with unsafe content. When online, WD uses 'Block at first sight' feature, which has malware fingerprints long before the signatures will be created for them.