(obviously this pcb not realistic, it is just to help my question)

"So a bigger ground plane would only help EMI and heat (which are very important)." So what is your reason to **not** make it all one ground plane? An abstract diagram like this does not show what may be happening in some other areas of the circuit. In circuits with fast edge transitions (it is edge speed, not pulse repetition frequency that really matters in Signal Integrity) the return current during the transitions follows the lowest impedance path back to the source which is essentially directly under the signal trace that the transition is travelling along from the source to the destination. Are you really sure that there are no signals with fast edges somewhere else that have to cross from one of your ground plane regions to get to back to their source? Bear in mind that the return current does not all flow directly under the trace, depending on the edge speeds involved, it is distributed over several trace widths either side. Therefore, the smaller you make a plane area the higher the impedance it represents and so the more likely it is that some of that ground return path current will still exist (i.e. will be non-zero) at the edges of the ground plane and so can cause crosstalk and radiated emissions. In my experience, there is never a good reason for breaking up a ground plane into smaller areas. There has to be a very particular reason for doing that and in most cases the "reason" breaks down when you look closely at where the return currents really are and where the traces have to be routed to make it work. :)

Thanks Andy, I will correct that ASAP!