Optimizing them for speed makes them flexible: when they're not full, you can go fast, and when they're full, they can degrade gracefully to 30-35 mph.

In London, the M25 (outer ring road) has variable speed limits. Perhaps it's to improve throughout on the M25, or to increase the capacity of the M25 to absorb traffic from nearby roads.

That makes sense, since they're not often near capacity between cities.