source?
“Incidence rate or person-time rate is a measure of incidence that incorporates time directly into the denominator” so speed seems appropriate https://www.cdc.gov/csels/dsepd/ss1978/lesson3/section2.html#:~:text=Incidence%20rate%20or%20person-time%20rate%20is%20a%20measure%20of,cases%20of%20disease%20is%20documented.