Top speed isn’t mobility.
Mobility is acceleration in forward, reverse, and traverse with a loose tie to top speed.
This is why Porsche Tiger has more mobility than Black Prince, and Tiger 1s have even more mobility than Porsche Tiger.
Also portions of the Porsche Tiger’s front are only ~150mm thick effectiveness.
Just over half of its hull front plate is 206mm thick.
Edit:
Porsche Tiger:
160mm of pen.9.6 base reload.
150 - 200mm of hull armor, ~150mm - 200mm turret armor.
Mobility: 0 - 19kph: 3 seconds [12 to stop speed] forward, 4 seconds reverse [13 to top speed].
Traverse 360: 35 seconds.
Top speed: 31kph both.
Tiger 1:
160mm of pen. 9.6 base reload.
100mm of hull armor, 130mm - 200mm turret armor.
Mobility: 0 - 19kph: 3 seconds [11 to 31kph]. 0 - top speed reverse: 2 seconds.
Traverse 360: 28 seconds.
Top speed: 40kph, 7 reverse.
Black Prince:
260mm of pen. 7.1 base reload.
152mm armor, 152 - 230mm turret armor.
Mobility: 0 - top speed: 6 seconds forward, 1 second reverse.
Traverse 360: 22 seconds.
Top speed: 18kph, 7.3 reverse.
More to be added later.
KV-220:
130mm of pen. 9.6 base reload.
120mm of hull armor, 100mm turret armor.
Mobility: 0 - 19kph: 2 seconds [9 to top speed], 1 second to reverse.
Traverse: 12 seconds.
Top speed: 34kph, 6.2 reverse.
Clearly the most mobile out of the 4 listed above, which makes up for the lower performing gun and similar armor.
IS-1, best mobility of the heavy tanks listed, slightly more pen, 37kph top speed fwd.
No testing required for this one.
T1E1 90mm:
180mm of pen. 9.8 base reload.
90 - 100mm of hull armor, 90 - 105mm turret armor.
Mobility: 0 - 19kph: 3 seconds [13 to top] both.
Traverse 360: 32 seconds.
Top speed: 37kph both.
Black Prince trades top speed and mobility based on armor thickness and penetration advantage over other heavy tanks.
There are of course heavy tanks less mobile than Black Prince elsewhere: Maus, TOG 2, Char 2, Churchills… however none of those are comparable to Black Prince due to either being significantly superior [Maus] or significantly inferior [the rest] in all other aspects.