Joiplay players rejoice, I found the source of the lag.
The plugin "BattleSplashFade" is the plugin responsible for the screen shattering effect, so disable that one if you want already, however it's not the main culprit.
The real cause is another plugin called "SceneFormation" (ver1.08), disable that one and all lag during battle disappears. I don't know what it's supposed to do though so if battles get fucky that might be because you disabled it, although I haven't noticed anything so far myself.