It's both actually, if you have a strong enough PC then there is no lag at all but you should NOT need one to run an RPG Maker game with Koikatsu animations, it's a matter of how RPG Maker wasn't meant to have these things embedded into it so it overtaxes the PC's memory way beyond what a 'game' like this should, sadly. It's not something that can be 'fixed' because it's how the game itself is programmed.
I found the plugin for that so if you want to turn it off follow those steps
In www->js folder
Make a backup of Plugins.js
Open it with a text editor and search for [SoR_GabWindow_Left]
You should get this:
{"name":"SoR_GabWindow_Left","status":true,"description":"Gad对话窗口-左 <GabWindow Type-Left>","parameters"
Then the ,"status":true replace [true] with [false]
Like this:
{"name":"SoR_GabWindow_Left","status":false,"description":"Gad对话窗口-左 <GabWindow Type-Left>","parameters":
Save and run the game
But you may miss indication on what to do since you won't get the dialogue at all
Why i'm not sharing the plugins directly it's because of the modding.