When the Fontself extension starts lagging, this can be due to a couple factors: your font glyphs may contain too many points/pixels, or Illustrator/Photoshop needs a refresh.
- If your glyphs contains complex shapes (like hundreds of points each), you might want to simplify your objects' paths to save memory.
- If you've been working for a while with Fontself, it may happen that some of Illustrator's internal processes (called CEPHtmlEngine) eat up too much memory. In this case, quit Illustrator & check that no CEPHtmlEngine additional process is running in your Activity Monitor (on macOS) or Task Manager (on Windows) - just mind that other Adobe apps may also run their own CEPHtmlEngine processes.
- Restarting your computer can help also to clear any memory leak and boost all apps again.
- Clear the text previews in the main preview or the Spacing / Kerning views. Long sample texts may consume much memory and processing power when you are dealing with complex fonts, and all the more when editing OpenType-SVG color fonts. Just select all previews and type the minimum of text you need.