Yes there is still the risk that it will run slower. However, you can still play in full screen. What you can do is in the game options. Go to the graphics tab, which should be open by default when you look at the options.
Go to Screen Size, and from the drop down menu, choose the screen resolution of: 1366x768. This will help the game run smoother.
Also what might help is to uncheck Advanced Rendering and Animation Smoothing. And check Enable Object Hiding.
Turn Off Interactive Loading Screens -- Go to Options (...) Select the gear tab, which is the General Settings, at the bottom of the menu uncheck the box labelled Interactive Loading Screens
Turn Off Memories -- Go to Options (...) Select the gear tab, which is the General Settings and look for the word Memories and in the drop down menu select Disable Memories.
While your in this tab, it may help to make sure Enable Shop Mode and Lessons are un checked.
Turning off memories means less game bloat/overly large game save file.
Clearing game cache every now and again helps
MAC - Clear your Cache Sims 3
http://help.ea.com/en/article/how-to-clear-your-cache-for-the-sims-3-on-a-mac/Here's a topic of links that might be helpful too.
MAC Users - Helpful Links
http://www.carls-sims-4-guide.com/forum/index.php/topic,20731.0.htmlMrsFlynn