Thanks for the info.
From the info you provided I can't tell if this is the correct problem, but if you use //..url.. (starting with double slash) and use Internet Explorer for preview (instead of Chrome), and you use files for preview instead of local webserver - Internet Explorer is dumb enough to try to resolve //url to a network drive that doesn't exist and this is what takes ages. So to fix it eliminate the circumstances - either use Chrome instead of IE for preview or get rid of // or use local webserver and configure mappings (menu Tools -> Options -> Preferences -> Preview -> Mappings) to map the file you're previewing to local webserver address.
Nonetheless, I'll file the CSS formatter bug for fixing, thank you for reporting.
Blumentals Software Programmer