I’ve watched the video record by you. For maxima session, I’ve noticed the problem. And for typing in other area, it might be the font issue. I removed several ttf fonts which is bundled with TeXmacs/Mogan v1.1.x, and the style files may still using those fonts. It might be the reason of the lag.
Here is the list of fonts:
If your document are using these fonts, it may cause the lag because when Mogan can not find the font, it will try hard to find it. Before when it try hard, it will scan all paths for truetype/opentype, and I’ve just optimized it in v1.2.1. But it will still scan all paths for TeX fonts.
If you can provide more clues on the lag, I will fix it in v1.2.2.