In addition to Sigil's program directory (not just Sigil.exe), you may want to try exempting Sigil's preferences directory as well.
In the past, I've seen Windows compatibility settings attach to Sigil.exe and slow things down terribly. I've also seen INI corruption slow Sigil down considerably (just recently, in fact). Sigil took forever to open/close/run until I backed up my preferences directory, deleted it and let Sigil create a new one.
But I do weird things testing Sigil, so INI corruption is par for the course.
