Games lag - unless screen recording is on

[technoviking]

Hi guys. I've got a two-weeks-old iPhone XS (bought new from an Apple store) on iOS 12.1 facing a strange issue. I've been playing a lot of Girls' Frontline and noticed that the game lags and stutters a lot during combat. I mentioned it offhand to some friends and they said that it runs perfectly smooth for them so I thought I'd take a video to demonstrate but oddly turning on screen recording instantly 'fixes' it and the game runs at full speed. I can turn screen recording on and off while I play and game performance goes up and down instantly. I'm guessing it's going into some kind of high-power mode or something when it starts recording. Is there any way I can set this to be on all the time?