Endurance is generally because of the batteries. They are of low quality and I never use them. Right away I purchase something more powerful such as the Venom 3600mah sticks (3+ hours enduarance). Now as for speed, the motors and gearboxes are the same, take a look at the drive sprockets, notice the difference? That's why the Tiger and Panther are such runners compared to the Pershing and Walker.

A note about the gearboxes. The gears usually need chamfering to get them to mesh properly, get the binding spots out. Even after I do that I still run the tank off the ground at varying speeds for 45 minutes or so to break it in.