The game runs flawlessly when I play offline against bots or when I start my own server, but whenever I join one my framerate drops to around 30-40, making it very difficult play. Same happens when my friend tries to join my server, it runs fine for me but it runs poorly for him. I couldn't find any solution to this online so I'm hoping someone can help me here? Thanks.

PS: Running the game in OpenGL

That sounds very odd, what version of the OpenGL renderer are you using??
If your using the version the game ships with you might want to try a newer one or D3D9 renderer and see if that makes any difference.
http://www.cwdohnal.com/utglr/

I was originally using the version of OpenGL the game comes with.
I tried the newer version of OpenGL from the site you gave me and I also tried D3D9, same result.

Hmm so doesn't sound like a renderer issue.
When you hosted a multiplayer game did you run it as a dedicated server?
You could try running a dedicated server and then joining that and see if it makes a difference.

If you were able to you could try connecting UT over LAN to see if that produces similar results.

When your friend said it ran poorly for him is that only when he joins your server and its fine at all other times?

Just checking that you are running version 436 (this is what the GOG version comes with) and haven't upgraded to 451 (this should only be used for servers)?

It looks like there's no issue joining a dedicated LAN server, I'll have to check and see if that fixes the issue online.
The issue persists in any server we join. And I haven't updated the game, I'm running the default GOG version.

Welp, nothing I tried fixed the issue.