This might have to do with a bug in the Unity engine that we are unable to fix, but that Unity will soon issue a fix for. If you set your resolution in the game to, say, 1080p, then you switch applications, when you return to Fantasy Strike the engine can secretly, silently set the resolution to the computer's native resolution. Mac laptops have very high resolution screens so this might set the game to an absurdly-too-high resolution and result in extreme slowdown.
To check if this is happening, just go to video options in the game and make sure it STILL says 1080p or 720p and not some enormous resolution.
We will patch this issue as soon as Unity issues their fix.