Welcome, @Habanero , to the OEM forum.
Basically - we don’t know, because the only substantive data is that which we’ve found out by dismantling a unit and reverse engineering it. You can find what we know here: First try with EmonPi - Pulsecount stuck at 1 - #16 by Robert.Wall
Also Adjusting sensitivity of Optical Pulse Sensor - #21 by Kempson
The sensor is a photodiode, speed will not be a problem for that, nor for the transistor circuitry (at least, not at the 70 Hz sort of speed you mention). The problem will be in whatever you are using for counting the pulses. If you will be using an emonTx or an emonPi, then the pulse counting is interrupt-based, but the processor is doing other things as well as counting pulses, and interrupts are of necessity turned off for short periods. I seem to remember testing the sensor by flashing a LED under software control, but I cannot find a record of the result.
I have a feeling that 70 Hz might be possible - but no evidence to support that. Again, if you are using an emonTx or an emonPi, the standard software incorporates an anti-contact bounce delay that means it refuses to count at faster than about 8 Hz, so that will need changing.