Note that CPU usage does not cross 80% in task manager during the entire run of six instances of matlab.
The processor seems to handle 6 instances of matlab because is not getting 100% CPU usage in task manager. I would recommend waiting for any other community member that may have experienced same behavior with same processor model.
I have timed running an iterative process in matlab in a loop. When 3 instances of matlab are running, each call of a function takes 5.5 -6 seconds in all three instances of matlab. When 6 instances of matlab are running, each call of function takes 7.5-8.5 seconds in all six instances of matlab. In both cases, the cpu usage does not cross 80 % and I am not running any other processes when testing this.
If we consider that one matlab instance is a single threaded process and you are running 6 of them, then two instances run on one core. So 2 cores have to do double the work, hence the bottleneck.
Two of the 8 threads are sitting idle, so this means about (100% / 8)*6 = 75% max usage which is about the same you are seeing.