Now I'm even more confused. There's zero reason it should be lagging. Lets try to Force-Update your nwjs ~
Or direct link (will likely be outdated soon though) https://dl.nwjs.io/v0.82.0/nwjs-sdk-v0.82.0-win-x64.zip
Download the SDK version, then go to Steam/steamapps/common/RPG Maker MZ/ and there should be a folder called "nwjs-win" Drag/drop the new files from the zip to this and then try (remember to make a backup just in case)
My guess is it has to do with the CSS styling I decided to go with, but even then, there's absolutely -no- reason it should be this laggy