Since CC 2018 Illustrator was released a significant number of problems reported seem to have been the result of not having "Import previous settings and preferences" unchecked in the Advanced Options which are offered during the install (see screen shot):
If you had that option checked during your install and have been experiencing problems such as speed issues, other instances of poor performance, or functions not working as you would expect them to then manually making sure that all previous preference files for Illustrator are deleted (or if on a PC renamed) may take care of your problem. After implementing one of the methods described below (making sure that InDesign is closed when you do so) you can then open CC 2018, save a test file and then quit it so that a new CC 2018 only preference folder will be created. (This is all based on a test I did that showed that when an older preference file exists CC 2018 did not create a complete preference folder). At this point I would restart the computer and once rebooted launch Illustrator and hopefully your problem will be solved.
To delete old preferences:
For Macintosh Users: With Illustrator closed launch a Finder Window in column view and click on your home folder. With the Option Key pressed choose Library from the Finder Go Menu. Within the Library folder find the folder called Preferences and within it find the following two files and delete them: “Adobe Illustrator <Version #> Settings” (earlier versions of Illustrator might just say “Adobe Illustrator”) and “com.adobe.Illustrator.plist”. When Illustrator is next launched it will create new preference files and the program will be restored to its defaults.
For Windows Users: You can try the quick way of resetting on a PC which is to hold down Ctrl + Alt + Shift when launching Illustrator and respond affirmatively when asked if you want to reset. There have been some recent reports that the window asking if you want to reset is not popping up but that the prefs are being reset anyway. If this works then great but if it doesn’t you may have to manually delete them.
On Windows 7 and above the preference files are hidden. To find them go to the Control Panel and open Folder Options and then click the View tab. Then select “Show hidden files and folders” or “Show hidden files, folders or drive options” in Advanced Settings (you may be able to skip the previous steps with Windows 10). Then delete (or rename) the folder at the end of this path: C:\Users\[User Name]\AppData\Roaming\Adobe\Adobe Illustrator [version number]\ Settings\<Language> for any versions that you have. Make sure that Illustrator is closed when you do this. When you relaunch the program it will create new preference files and the program will be at its default settings
After you’ve reset up the program (make sure that no document window is open) to your liking, it is a really good idea to create copies of your personalized “mint” preference files (make sure that you quit the program before copying them—that finalizes your customization) and use them in the future to replace any corrupt versions you may need to delete.
April 2018 Update to the above post: According to recent reports, it is possible that previous settings and preferences are being imported regardless of whether that option has been checked. The information in the above post should be helpful in dealing with this issue no matter how the previous preferences were imported.