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.
Disable Clipboard History on Windows 11/ 10 from Power menu > Settings Q System > Clipboard and restart your computer.