You are experiencing a known bug in the new user interface of
Windows 11.
The new Command bar in Explorer is leaking memory furiously and
causes explorer to slow down progressively as time passes and more
memory is lost.
The test for this is to use Explorer in full-screen mode by pressing
F11. This will return the interface to classical Windows 10,
and will especially remove the Command bar and liberate its memory.
If the lag is gone in full-screen mode, then you are a victim of
this memory leak bug.
A temporary solution is to restart Explorer therefore returning to the
situation as after you booted. See the article
How to Restart explorer.exe Process in Windows 10.
A better solution might be to use an Explorer alternative.
For a plethora of choices see the article
14 Alternative File Managers To Replace Windows 10 File Explorer.
My personal favourite is Altap Salamander, but this is a matter of
personal taste.
In the meantime, keep Windows fully updated.
This bug is not new but Microsoft has not yet fully fixed it,
although some improvements were done. Microsoft might in the future
finally get it right...
