Right after upgrading my Macintosh laptop to Lion I found that iCal, the Mac calendar, was so slow.  It was really aggravating.  It happened with every change I made to my calendar.  I guessed that the reason was my calendar was uploading to a remote server.  I did have a .Mac account so I decided to look for this cause.  I did find it to be the case.

Solution:

• Open iCal preferences.
• Choose Accounts.
• If you see any listing showing “CalDav” select it.
• Select Account Information.
• Note the line listed as “Refresh Calendars:”
• I found this to be set as “Push”.  THAT IS THE PROBLEM.  Change it to anything else, but not “Push”.  I chose every 15 minutes.

This completely solved the problem for me.