I also have very slow auto-save here. It goes up to about one minute.
Loading and saving is faster when the song is just loaded. So saving, closing Dorico, open Dorico and loading the song again shortens the saving time to a normal amount of a few seconds. But it gets slower the longer I work and after a few hours I’m back to the long saving time again. If Dorico is running playback and the saving process starts I can just wait until the song plays half through. The spinning wheel on Mac shows up and all input from the computer keyboard and mouse is blocked while saving.
I use external VST instruments but the songs I’m working on are combo arrangements (10 tracks) and the song length is about three minutes. So not really big projects and very CPU easy VSTs. The Dorico file size is below one MB. Saving a song with exactly the same instruments in Logic takes less than a half second.
I remember the slow loading times in Dorico2. As far as I remember it was going better after the Dorico3 update. Now it’s back. But I can’t say exactly when.
I’m on a High Sierra 8 core MacPro (2013) with 96GB Ram and SSDs. I can run the same songs on my 2011 MacBook Pro i7 16GB Ram Mojave without problems.
I don’t have the long saving times on the MacBook! On the MacPro I use a dongle, on the MacBook the software licenser (I have two Dorico licenses).