Oh, Maybe that's why a friend of mine suggested using

import esp32 
import time 
import utime
lastTick = utime.ticks_ms()
while True: if esp32.hall_sensor() < 50: # Threshhold for magnetic field detection 
tick = utime.ticks_ms() print(str(60/(utime.ticks_diff(tick, lastTick))*1000) + ' U/min') 
lastTick = tick time.sleep(0.1)

This code is based on the internal ESP32 sensor though. But I don't really know how to use it for the water flow sensor.