The achievable clock speed is mostly a function of the pipeline depth and how well the design is optimized, the ISA should have very little impact on it. There

The achievable clock speed is mostly a function of the pipeline depth and how well the design is optimized, the ISA should have very little impact on it. There are a bunch of high-clocked high-performance ARM chips available because several huge companies have already made large R&D investments into developing them.