Several possibilities...
In Logic's Preferences....
Perform all tests, one at a time, close and reopen Logic after each attempt. If there's is no improvement return to the former setting and try the next test. Set you audio interface settings in Logic, do not select your interface in the Mac's System Preferences and then select system settings or whatever it's called in Logic. Set your interface directly in Logic. Is your Audio/MIDI interface 100% compatible (drivers) with the version of OSX you're using?
1. Under the Multithreading Option... Set it to "Playback Tracks".
2. Under the Processing Threads option, click and hold, select 1/2 the maximum number of threads available.
3. It could be a controller issue causing a drag on the system from an internal MIDI loop.
With Logic not running Delete the Logic 10 preferences file AND the shared control surfaces file.
(for now only delete the control surfaces file)
In the Finder, choose Go > Go to Folder from the menu.
Type ~/Library/Preferences in the "Go to the folder" field. Note: Type/cut paste this---> ~/Library/Preferences
Press the Go button.
Remove the com.apple.logic10.plist file from the Preferences folder. Note that if you have programmed any custom key commands, this will reset them to the defaults. You may wish to export your custom key command as a preset before performing this step. See the Logic Pro User Manual for details on how to do this. If you are having trouble with a control surface in Logic Pro. then delete the com.apple.logic.pro.cs file from the preferences folder.
You can also try deleting Logic's preference file, you'll have to go through preferences and reset your settings.
