If you're joining different servers and this issue doesn't show up, it can't be only client-side because this issue isn't being persistent across multiple servers. If our ZS server is the only server you are experiencing a drop in FPS due to prop-rotation, then it sounds more like a server-side problem.
Unfortunately, I can't say there's a server-side solution to fixing this because users that experience framerate lag on our server have it for different reasons, and your case isn't unique in that regard. This is overruled in the instance that a currently-played map or something on the map is creating lag, in which case everyone on the server will lag.
I also would say that this isn't entirely because of the files you have in your Garry's Mod directory, but if that's the case, then the problem is part client-side, part server-side. However, I think the issue you're dealing with is server-side @SoFast142.
If for any reason the framerate drop is because of your files, you need to unsubscribe and completely clean all your mods from your addons folder in your Garry's Mod directory. After that, you're going to need to verify the game cache to make sure all the correct files exist in the directory and that nothing is out of place.
As a last resort, if the situation is still not resolved, you could try uninstalling and reinstalling the Garry's Mod client game entirely and start from scratch, but as I said, this is a last resort. Uninstalling may get rid of issues, but it could also bring up new ones.
So, you should've been more specific with this because binding can be accomplished in two ways.
Option 1:
In Garry's Mod's menu screen, click on "Options."
Find and click on the "Keyboard" tab.
In the keyboard tab, double-click on the action you want to set a keybind to.
You'll be prompted (signified by an empty orange box) to enter a key to bind the action to. Press your Alt key, and that action will be binded to your Alt key.
you can only bind one key to one action at a time.
Option 2:
In you're Garry's Mod's main menu screen, click on "Options" and press ` (tilde key), or whatever key you have opening the console set to, to open the console.
Depending on what you want your Alt key to do, you'll type: bind ALT "<key command goes here>". Refer to the list of Garry's Mod console commands here by Outerspace2018: GMOD Console Commands Guide
the player action you want to be executed by pressing the Alt key goes in the quotations.
Ex.: if I wanted to bind my Alt key to allow my player to jump I would type into the console: bind ALT "+jump"
Close out of the console.