I've been playing with a few mods of mine in NTT (a couple, more powerful versions of regular shell weapons, nothing too special) for testing balancing, and when i got to about L1 labs, the game slowed down to about 5 FPS (though regular NT and other NTT levels run smoothly), and in some other ocasions, testing levels sizes with all the levels, the sewers, frozen city and labs seem to lag the game waaaaay more than regular areas (which still lag), and i guess that might be because of level generation creating huge corridors. Is this fixable in any way?

I don't recall anyone reporting framerate issues with labs. It could have coincided with something like Steam client glitching out (which causes the mod to lag heavily because it has to poll it's events to be able to do networking). Restarting Steam usually helps that.

If mods are written suboptimally (e.g. picking through every enemy/projectile/wall every frame), that can pose it's own problems. That would be a thing to investigate separately by loading/unloading mods and watching how framerate changes.

In general though, the more walls\floors\enemies\projectiles there are, the slower the game gets.

This is usually helped by switching to spatial collision system introduced in GameMaker last year, but I am unable to make use of that in NTT because it's currently not deterministic, meaning that it noticeably messes up networking and breaks replays entirely.

Since the option cannot be toggled while the game is running, the only way to utilize it for local sessions would be to make a separate "offline" version of the mod, which isn't something I'm enthusiastic about (spending time adding conditions to everything and then spending twice the time to make every new build).

Overall, mostly can only wait till things are fixed on GameMaker side.