Windows Defender is making Windows 11 unusably slow

Try to use Process Monitor by Sysinternals to look at what it does exactly at the moment of slowness:

https://learn.microsoft.com/en-us/sysinternals/downloads/pro...

I believe you can exclude files / folders from scans.

Is it overall slowness or specific apps? You might want to exclude say, builds where there's a lot of processes started.

https://learn.microsoft.com/en-us/defender-endpoint/microsof...

I am on w10 and i have ripped out entire defender and all that security crap. i've been online for quarter of a century and never caught a virus or got hacked. all these security features are waste of hardware resources. i can easily see this being much worse on w11.

Modern OSes constantly indexing all your files is not a bug, it's a feature. If you want your alternative you can install it from a live USB at your earliest convenience.