Many reasons, to help narrow it down create a new document draw one lement and let us know if you have a slowdown.
Reset your preferences.
Cmd-Opt-Ctrl-Shift when restarting AI on a Mac or Alt-Crtl-Shift on a PC to reset to the defaults.
Flush Cache (Good for slowdown)
Fo you have alot of fonts loaded. You may have a corrupt font. Close any you do nto use often
Run applications >> utilities >> disk utility and repair permissions.
Run applications >> utilities >> system profiler and check the status of your ram and hard drive
If the test you did in the beginining returned your computer to a speedy apce, search the forums for HUH error.
Download onyx from apples site for your OS version and do the cleaning tab.
Thanks for all the work you put into replying to the OP... I know it was a while back.
However I worked all the way through your list to no avail... then found another thread (http://forums.adobe.com/message/3728652) recommending to uninstall RightZoom. This fixed it straight away