450 pulse/h is no problem for a input card.
A PLC can lose input signals from flow meters if the pulses are too short or less than scan time. So if the pulse is shorter than the scantime in the PLC you have a problem
I have experienced this on a project where the pulses were shorter than the PLC scan time so we lost the measurement. The solution was to mount Weidmuller MCZ timers on 150ms on signals. Weidmuller MCZ timers available in both 50 and 150 ms version.