So I set my CPU to 100 MHZ. At that point the router can route + nat about 10 megabit (I can go to about 40).
Strangely, the CPU indicates it is being used for about 30%. Same usuage then when it is on 680 MHZ.

So I could conclude the CPU is not being maxed out at 100 MHZ but the fact I can only get 10 megabit tells me differently.

Is this a bug?

For anyone wondering about this in the feature, here is the answer from support:

CPU freuency and usage is not proportional to throughput, never was and never will
be.
Memory throughput, internal bus speeds, RouterOS configuration also have impact