Computer starting acting very very slow.

Excerpted from Use safe mode to isolate issues with your Mac:

If the issue continues in safe mode

If the issue continues while in safe mode, or your Mac restarts several times and then shuts down while starting up in safe mode, you should reinstall macOS and make sure that your Apple software and third-party apps are up to date.

...

If you still need help, please contact Apple Support.

Most likely, a hardware fault is indicated, and the most common reason for that is startup disk drive failure.

Same thing for me, it started happening an hour ago after installing the supplemental update 10.15.7 (19H15). I have the same macbook pro model than OP.

I restarted my machine multiple times but it still laggy.

Yes it's ok. The ~yourusername is a temporary file created during installation, and should be deleted by the installer.

Please determine if the same problems occur in "Safe Mode": Use safe mode to isolate issues with your Mac.

👍