Just an update...
I tried a few things to fix the problem including disconnecting my computer from the internet and booting in safe mode to see I could close any tabs or clear the cache or anything, but that still didn't do anything to help, so I finally decided to do a full uninstall of everything and reinstall Opera.
Firstly, my user data backups didn't work and I lost about 2,600 tabs, all my bookmarks, and multiple important workspaces that I use for work while doing this, so I'm absolutely gutted, but secondly, even with only two tabs open Opera's still freezing, except it's taking about 20-30 seconds instead of 2-3.
I wish I knew what caused this in the first place, 'cause like I originally said, one day it was working absolutely fine and then the next it was completely stuffed. No updates to Opera or Windows in-between or any new, unrelated program installations or anything. This was brutal.