The clock speed is, today, not a very good measurement, the instruction cycle time is what you should consider.

As an example, let us take a lowly (<$1) LPC 932
The max clock speed 12MHz, so it does no fit your requirement at first glance, but since it is a two clocker the instruction cycle time is 6 times faster that a 'standard' '51 equivalent to 72MHz.

If you want dramatic speed, let us consider a Cycgnal running at 96MHz = 8 times faster. Since it is a one clocker, there is an improvement of 12 times, so the result is that the equivalent is 12*96 = 1152MHz = 96 times faster compared to a 'standard' '51.

Erik