Mike , Yeh , its never espressed in seconds , as its pulses per inch.
It has to be derived from frequency however - as it has to be dependent on speed of travel.

What Im wondering is .. we all realise that headspeed is never really constant ..do the machines that quote PPI actually vary freq continually to compensate for this?

In the case of a machine that allows you to set freq alone , the pulses per inch using that freq would be related to head speed. So a user would have to know head speed to set the correct PPI to avoid or minimize charring.

At the end of it all , getting back to the original question , the obvious answer is that the low powered laser is the wrong tool for cutting thicker wood if you want clean edges.