These days CPU clock speed is actually kind of irrelevant to the overall speed. It is possible to design vastly more efficient chips at the same clock speed, so you could have two chips both at 1 GHz, and one runs twice as fast as the other!
Rendering in this case is probably more determined by memory performance than clock speed, so maybe their memory specs are the other way round there.
The Android browser is also extremely inefficient, even on Android 4. It does not use the graphics card to draw. Try the game in the CocoonJS launcher
and see if it runs any better - it uses full GPU acceleration and can be as much as 5 to 10 times faster.