I guess when number of row by seconds is superior to a certain amount and total operation lasts less than one second, 0 is displayed.
I suppose TOS uses System.currentTimeMillies() to get operation durations, which gives millisecs so over a certain speed, the program cannot see the difference between the beginning and the end of operation and in order to avoid a "divide by zero error" it gives an automatic 0/row by secs which is in fact an "infinite amout of rows/sec to our precision".
Cheers,
Diane