But that's not entirely correct either. Yes you can use it like that, but you can also use it as a single core with a vector length of 1536.

In the context of over-simplification these are better thought of as single-core processors. The reason being if you have method foo() that you need to run 10,0000 times, it doesn't matter if you use 1 thread, 2 threads, or 8 threads - the total time it will take to complete the work will be identical. This is very different from an 8-core CPU where using 8 threads will be 8x faster than using 1 thread (blah blah won't be perfectly linear etc, etc).

The GPU is creating threads and tasks internally and it's not always easy to balance this workload so no parts of the GPU becomes saturated while following parts in the chip's pipeline are idly waiting for work.

The PowerVR chips we're working with have dozens and dozens of different profile metrics corresponding to the different areas of its pipeline, each one being a potential bottleneck.

You could do something as silly as render a ball with 12k vertices instead of 24 and expecting the vertex processing to be much slower, but after profiling you find out its the fragment part lagging way behind because the data sequencer is overloaded trying to generate fragment tasks. In both cases you're rendering about the same amount of pixels.

With unified shader architectures, its very frequent for vertex and fragment tasks from different draw calls to overlap simultaneously. We're even seeing tasks from different render targets overlapping! Such as fragment tasks from the shadow pass still running when the solid geometry pass is processing its vertices.