I am sure it has to do with the number of packages.  I have watched the time to go up.  I can remember when it used to take 5-6 minutes.  Keep in mind that I have not really used v285, and I have barely used v284.  I usually run a number of things at a time because I am a music junkie, so iTunes is always running in the background, and Firefox or Google Chrome is open because I am on the forums or working on websites all of the time.  I can get a more official time when I get home.  The 10 minutes things is a ballpark number, but it is somewhere around there.  I do not mind.  Most people have little to no reason to have over 700 packages in their Installers folder.
Edit:  The combination of unreliable memory, the v284 update and me closing 
most things makes the 1:35 stops my excitement from sending me out of this chair.  I seriously used to go do something else while I let the Installers tab load.  That is just awesome!  I wonder if it still takes ~6 minutes to rebuild the Bashed Patch for a 300+ mod setup.
Edit:  By the way, my list is currently 864 packages strong, but now that does not bother me at all.  I care about the load time, not the package list length.  The package list is sorted and labeled.  ^____^