iMac Speed

Close all other open apps. Make sure you are not using any anti-virus apps, cleaning apps, or third party security software -- none of which are needed on a Mac anyway and which usually causes performance issues.

Have you adjusted the games settings such as lowering the graphical details and disabling other graphical items such as anti-aliasing, shadows, etc. and even lowering the games resolution?

Does your system meet the current Recommended System requirements for both hardware & software? If your system only meets the Minimum Requirements, then you are going to have performance issues even with all the games settings turned down. Keep in mind WoW's system requirements can change over time especially after a new expansion or major game update is released.

Also try disconnecting all unnecessary external devices in case one of them is causing a problem.

Have you check the WoW game forums to see if any other Mac users are reporting any issues especially with the same hardware and OS you are using? Games are always very hard on a computer and usually you will only find answers from other users on the game and you will find more of those users on the game's own forums.

You can also contact Blizzard for assistance.

If your Mac operates normally with everything else, you can't.