How to avoid big lag spike when alt-tabbing out/in of engine

It is quite jarring how impactful on fps it is to alt-tab out and in of the game.

As a follow-up question, anyone know how to trigger borderless window from unrealscript (which may avoid this issue altogether)?

you mean windowed mode? because fullscreen applications have always caused issues when alt-tabbing. if you mean windowed I don’t really have such issues
borderless does not exist in UDK btw

I’ve never seen a UDK or UE3 game run in a borderless window. I’m not saying it can’t be done. I just haven’t seen it.

This is Chivalry forum post that explains how to do using launch params: full screen borderless windowed mode? EDIT: Solved :: Chivalry: Medieval Warfare General Discussions

Just wondering how this achievable through unrealscript.

did you actually get that to work in UDK?
I’m pretty sure Chivalry implemented it themselves in the engine. we did the same at work in our old UE3 project