Here is an article from Maxim on modular exponentiation, modular arithmetic accelerator (MAA) and lists typical times to execute various sized exponentiations:
The MAXQ1050, MAXQ1850, and MAXQ1103 are secure 32-bit microcontrollers that provide hardware support for performing modular arithmetic. This is done using an engine called the modular arithmetic accelerator (MAA). This application note gives typical execution times for various modulus sizes, key types, and optimization levels.
Interesting devices, but according to the Maxim website obtaining datasheets for them requires an NDA, and if they’re anything like other secure microcontrollers I suspect that they’re not exactly open to giving hobbyists access. None of the chips with modular exponentiation accelerators I’ve come across are exactly suitable for open hardware designs.