I have just installed and tested SSD 1.6.0.25 and find it a great improvement: many congratulations!
For the record, my system is running XP (Home edition) SP3 on a 1.7Ghz P4; 512 MB memory; Avira AntiVir free, Comodo Firewall Pro, Memory Firewall, and BOClean.
I have used SSD 1.5.2rc1 (=1.5.1.19) since January, averaging 27.5 minutes per scan, although increasing slightly over time, presumably because of added detections, although a change from AVG to AntiVir may also contribute. I tried 1.5.2, but was not pleased: it took around 50 minutes per scan (only two were done, though).
With SSD 1.6.0.25, my first scan was 19.5 minutes and about 4 minutes of that was scanning bookmarks (see below). Very nice improvement from 1.5.1.19 and a dramatic one from 1.5.2.20.
Now a suggested option: I have four browsers on my system (IE7, FF2, Opera 9.5, SeaMonkey) and go to some trouble to keep their Bookmarks/Favorites synchronized. There is surely no need to scan them all and, as noted above, it would save around 15% of the scan time to scan only one. So an option to select one browser only would be very nice.
I would also suggest making the Hint of the Day optional, except that others have already done that. I vote with them.