Hello thanks for the report. The lag is the loading screen I believe. The black screen is removed but the loading time is still their so it feels like the game lags. It can't be completely smooth because the loading needs to take some time. But it can be shortened if you use a plugin to cache the resources of the battle (enemies, background, etc). Besides that there should be a frame rate drop during the animation for slower computers,
If the loading lag is still too great for our game, maybe you need to find another transition plugins that keep the black screen.
As for battle test, I think I can start fixing it in one or two days.