Well, there are multiple ways to fix this problem but the only one that I think fully works is just simply clearing your harddrive. Harddrives that are almost full can cause slow booting, slow performance and random crashes in applications. If that does not work then you should go and delete all addons you have in your steam folder and so on.
How to do that is by:
firstly going to steamcommunity.com, log in
go to community>workshop, then scroll down to where it says "Your Workshop Files"
Now once you're on goto subscribed addons or whatever (the 3rd box) when here press F12, a little menu will pop up
Take and paste this code into there: (function(){jQuery('.btn_grey_black').map(function(){this.click()})})()
Do that for every page you have for addons.
Now, what you'll have to do is go to your harddrives.
Go to this file C:\Program Files (x86)\Steam\steamapps\common\Garry's Mod\garrysmod\addons
Once you are here, take and delete all files in here except for CS:S content if you have that.
Now, that should do the trick, if it doesn't then I suggest taking your computer to service to try and figure out what the problem could be.
If you have any questions then create a pm with me
here; or add me on steam
here.