Treaty speed limits for different capital ships

How could a treaty keep battleships and battlecruisers from merging into one category?

Set a maximum design speed for battleships and a higher minimum for battlecruisers, while allowing the same displacement limit.

A hypothetical treaty could cap battleship design speed at 28 knots and require battlecruisers to exceed 30 or even 32 knots. Both categories could retain guns and armour within an equal 35,000-ton allowance. The speed requirements would constrain what designers could achieve inside that displacement. The later London treaty treatment of sloops, cited here as limited to a 20-knot design speed, supplies a comparison.