Well, a few things spring to mind when seeing your mixer: first, Reaktor is a CPU-guzzler, especially in Live mode (= when its' track is selected). You also have quite a few instances of Kontakt, and 3 instances of Massive (as if it's named after its' CPU-impact). I cannot accurately assess which is the bigger contributor though, since the CPU load of all these very much depends on how they're configured - but each of those can easily be configured so they can overload any CPU.
So, if they already stress the CPU when playing 0 voices, chances are you're going to see CPU-overloads pretty soon, especially since you have relatively slow CPU-cores.
You should try to find out which of these instruments are the heaviest contributors, then reduce their load by either turning off any onboard effects (especially reverbs), or by freezing the tracks.
In your Audio preferences: when mixing, there's no reason to not use the 1024 samples buffer - this alleviates CPU-pressure. Also, set the Process Buffer Range (the mixers' buffer) to Large.
Logic Pro X: System Overload messages - Apple Support
