That's not entirely true. Driving wheel size is important, but you can
still run fast at a relatively low bp and on small cylinders provided you
can generate enough steam steam and deliver it quickly. A K class can run
at close to 100km/h (with small driving wheels) on only 175psi.
It has more to do with the efficiency of the boiler than the actual
working pressure. The size of the grate, good draughting, the thickness of
the water space around the firebox and the efficiency of the valves are
the critical factors.
Cheers
David