GPU GeForce GTX 1060 @ 3GB:
240x135 tile -> 1m30s
480x270 tile -> 1m25s
960x540 tile -> 1m36s
512x512 tile -> 1m32s
CPU Intel i5-7400 @ 3GHz (4 threads): 64x64 tile -> 6m26s. I gave up further CPU tests.
When rendering with GPU, Task Manager showed 30% CPU usage and 14% GPU usage on average.
When rendering with CPU, Task Manager showed 99% CPU usage and 1% GPU usage on average.