Heroes VII

I haven't seen a single unit with speed 3.

Arbalesters seem to have 3.

Especially considering that the turn frequency of units doesn't differ much either.

Yes, that's a drawback.

And by the way, I'm not against 7-8 speed – these speeds really add variety if they occur relatively rarely.

And they do occur. Air elementals, for example, seem to have a speed of 7 – they reach the enemy ranks.
Well, in general, you're probably right about the speeds, Nargoth. It's just... They seem to have taken the right approach again (limiting unit speeds), but the implementation once again turned out to be not very good (all speeds are too similar, which hurts variety). It's the same with everything.