I have minor lag problems from time to time. (By 'lag', I mean FPS/performance issues.)
It generally only happens in the third room of an arena or a danger room. If I turn my settings to low I tend to only get this problem when 10+ monsters all spawn at once, then it levels out and I can play fine.
However, I just cannot do FSC for some reason and I don't understand why. I think it may have something to do with that dungeon usually always being run with 4 people. The rooms with fire vents are particularly bad, I feel they may be lagging me out.
I literally got to a part where I was spamming Z to shoot my polaris and instead of firing individual shots I got stuck charging a shot until I got surrounded and killed by skeletons.
I've got settings on low with compatibility mode on to no effect.
The only solution left is getting a better PC. Sorry.
Or try to convince the developers to use friendlier methods and less fancy effects.