The posts are worth a read if you are interested in how to make your websites and web applications quicker.
I decided to run YSlow against a web application that is in the final testing phase - just out of curiosity. I'd already checked the performance with firebug and was pretty happy with the speed, especially seeing as it used a few of the dojo libraries and was much faster that the clients website.
I was disappointed when YSlow reported a middle of the range score.
I then ran YSlow against a number of popular websites and web applications, all of which fared similar to mine. Finally, I decided to run YSlow against the local Yahoo website au.yahoo.com.
Yahoo gets an F.
I felt much better after that.
After a bit of research, it seems as though you shouldn't take the score card too literally. There are plenty of popular websites out there that don't score highly according to YSlow.