I think there are two constraints the GPU would impose on the CPU: 1. Static resource contention—i.e. how many transistors are allocated to a task. 2. Power/H

I think there are two constraints the GPU would impose on the CPU: 1. Static resource contention—i.e. how many transistors are allocated to a task. 2. Power/Heat contention: assumably the CPU can run at full force without the GPU. When the GPU is also cranking, it's unclear how the processor divvies up the power. Optimally, it would not affect the other, but with so much money going into power management and conservation research vis-a-vis phones, it wouldn't shock me to find that it cut into the CPU significantly: I would suspect the GPU is a much more recognizable piece of quality hardware to most consumers. My Moto E is an impressive, cheap piece of hardware, but still chokes hard on lollipop animations.