I think this is the one of the simplest way.
N pieces of light barriers
if only one barrier is used, system gives lap times
Every time PC get pulse input, real time is stored.
Pc calculates time between pulses and
when distance between barriers are known , speed is calculated too.
Print time number , time and speed
loop this until stop
You can do recalculation with selected time number,
to get separate lists for part times , speeds and lap times.
etc...etc..