So, of course, you should be wondering why upgrading your Mac to Mountain Lion should be done on weekend.
If you are ready, I would schedule a time of upgrade and I recommend that you do this a few hours before you go to sleep.
This is because Mountain Lion upgrade is a "production interrupting" event, and it will actually not make your computer usable reasonably for up to several hours. In my radiology job, this is very customary, we do not upgrade any computers until all the radiologist are gone home.
After the computer boots up when the upgrade is applied and you log on, there are two things that are very time consuming.
- Some re-indexing activities like building of Spotlight and such. Many people have reported that Mountain Lion is "slower" but this is because initially it takes a while for this rebuilding activity to complete. So leave it alone and let it finish and it will be faster.
- As I wrote in another article, Mac Mail files must be migrated from the original location to the new "sandbox" location. This can take hours to complete in some cases.