I've noticed that the game slows down to maybe 1/5th speed whenever I do defense missions in the campaign. Upon killing the enemy's brain, performance returns to normal. Any idea what's causing the problem? I'm guessing it has to do with AI.

I haven't noticed that in campaign but I just did a skirmish and after a very short time it did what you are saying (basically slowed WAY down and stayed that way until a brain was killed).

Some sort of performance bug clearly.

EDIT: It actually seems to happen even before you start fighting, in the building stage before skirmish. Could be related to number of units, but I had barely any (like maybe 10?) and a small bunker and it still happened.

I've noticed that happening too on the new build.

Whenever I start a battle in Skirmish and the campaign, it slows down incredibly to like 10-20 fps (not exactly, just an estimate on the feel).

Its not because of the level of performance of my computer because it used to be perfect speed pre-B27.

It doesn't always happen, I played Dummy Assault and it seemed fine in B27. But yeah, it is a fairly serious issue when it does. Basically the game becomes unplayable and there is no way to fix it other than quit.

Very bad slowdown is usually caused by Lua scripts spamming errors- check the console (or the DebugConsole.txt after closing the game) to see if there are any problems there.

Certain scenes do seem a bit slower than they ought to be, though, even without errors.

Wasn't aware of that, I'll check it out...thanks.

Yea I checked the dev, it is spamming error messages.

Its happening in almost every scene, campaign and skirmish. So its the mods causing the problems right?

Most probably. A few changes were made to a couple of Lua functions, so some old scripts will be quite borked. Best just wait for an update to the mod in question.

Somehow this build is turning out to not be as good as I was expecting it to be...T_T