Just completed an upgrade to ML and I was totally dismayed at how slow my system was doing anything. I pressed Cmd-Space to open the activity monitor only to find that Spotlight was hung on "Estimating Index Time". After about 30 minutes of searching here and on the web I determined that this appeared to be a somewhat common problem with other OS X upgrades but no real solution. After about 45 minutes of it not progressing at all, I decided to try a shutdown and restart. Took a very long time for the shutdown to complete, but once it did the restart seemed back to its normal time and everything appears to be running OK now.