I think I understand, when you say group, approximately how many units?
RPG Maker has a problem with the playback of Effekser animations; a certain number of animations running on the screen result in FPS drops in almost all scenarios, depending on the type of enemies. It can be adjusted so that their attacks and collisions don't play Effekser animations. I performed a complete test of the demo after your alert, including recording and uploading the video to the page, and I didn't experience any lag.
But the plugin should work the same for everyone. If you're still experiencing lag, there's still room for improvement.
Thank you very much for the feedback, I'll perform some more tests and modifications! I'll let you know soon.