Closing this as
Sigil 1.4.3 has been released with several critical bug fixes. If you're using Sigil 1.4.0 - 1.4.2, please uninstall and install version 1.4.3 instead.
@FlaJunkie if your slowness (on the one machine) persists with version 1.4.3 please bring it up in 1.4.3 thread (or start a new one). I have some questions about your setup on the machine where Sigil seems to have a serious case of the slows. Whenever you get time.