From the docs:
By default, Query Monitor’s output is only shown to Administrators on single-site installations, and Super Admins on Multisite installations.
In addition to this, you can set an authentication cookie which allows you to view Query Monitor output when you’re not logged in (or if you’re logged in as a non-Administrator). See the Settings panel for details.
You should only see a slow-down as the admin, not as a regular user, and only if you’re logged-in. But yeah, if you don’t need it all the time, deactivating until you do is an option to keep your own performance up.
EDIT: As far as why, the plugin dev could explain in detail, but I would totally expect a plugin like this to create performance impacts. I’m guessing it’s hooking in and recording every step that WordPress makes on each request, and there are A LOT of steps.
-
This reply was modified 4 years, 6 months ago by positivejam. Reason: Added the "why" for the slow down