Very accurate time measurement
The speed of the microcontroller is not always the limiting factor. The MSP430 can be an appropriate solution, not because of the fact it only runs up to 25MHz, but because several MSP430 devices have the Timer D peripheral which enables up to 4ns time period (256MHz). This is faster than almost all microcontrollers out there. Even the STM32 in its latest versions (F4) can only do 180MHz.

Timer D is available on MSP430F51x1 and MSP43051x2 devices such as the MSP430F5131.

However, that only solves being able to capture the time. The big question is what do you do with it because the processing itself will be slower. You could sample the time between two pulses, but you won't be able to do any processing between them, if that's what you mean.