When I open a font package, Glyphs becomes very laggy or even freezes, but when I open a regular .glyphs file, it runs smoothly. I suspect it might be related to some plugins. I’m using Glyphs 3 (latest version on macOS).
Can you test without plugins? You can duplicate the “Plugins” folder (located next to the Scripts folder), then remove half the plugins in the original Plugins folder. Restart Glyphs. If it works again, put pack a quarter of the original plugins, etc., etc. until you’ve narrowed it down to the problematic plugin.
Can you try to start without plugins. Hold down the Option + Shift key when you start Glyphs.