Device performance can be affected by dynamic frequency scaling. For example, running long kernels on both devices simultaneously might eventually result in one or both devices stopping use of the Intel® Turbo Boost Technology. This might result in overall performance decrease even in compare to single-device scenario.
Similarly, in the single (Intel® Graphics) device scenario, a high interrupt rate and frequent synchronization with the host can raise the frequency of the CPU and drag the frequency of Intel® Graphics down. Using in-order queues can mitigate this.