I still don’t know how you’re processing the sensor output, so I can’t help all that much. But if the sensor is straight into the GPIO of a Pi Zero, then could you be following this: Directly connecting to Optical Pulse Counter with RPi? - #2 by pb66 ?
The output of the sensor is actually an over-driven analogue output of the sensor diode, as you can see from First try with EmonPi - Pulsecount stuck at 1 - #16 by Robert.Wall.
There’s nothing in there apart from the switching speed of the diode and transistors (and stray capacitance) to limit the pulse rate.
I couldn’t have reasonably guessed that - it’s still a rarity in the UK.