IMac slow and reboot takes 5 minutes after Yosemite

Question

I updated my late 2012 27" iMac from the latest Mavericks to Yosemite.  now it takes 30+ second between mouse click and action.  For example, if I click on iTunes it takes 30 seconds to a minute before iTunes opens.  I get the pinwheel while waiting.  I rebooted and the startup hangs on the progress bar for several minutes, then black screen for several minutes, then eventually gets to desktop.
what would be the normal steps to start troubleshooting this issue?  I am relatively new to macs and haven't had this type of issue before.
thanks in advance!

Answer 1

Launch the Console application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Console in the icon grid.
The title of the Console window should be All Messages. If it isn't, select
          SYSTEM LOG QUERIES ▹ All Messages
from the log list on the left. If you don't see that list, select
          View ▹ Show Log List
from the menu bar at the top of the screen. Click the Clear Display icon in the toolbar. Then take one of the actions that you're having trouble with. Select any messages that appear in the Console window. Copy them to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.
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