Other factors will be the F.O.C. of the arrow, how well the bow is tuned, size and amount of helical on fletching, and whether you are using fingers or a release.
I don' t think anyone can give you a number that will apply in all situations. In fact , the number is probably at least slightly different for everyone. I would also guess that given the perfect setup and equipment, the maximum speed is probably a lot faster than what we' re shooting now.
Interesting question...It wasn' t all that long ago that I read where Chuck Adams wrote that he felt 220 fps was about as fast as a person would want to shoot an arrow with a broadhead and still be accurate. In his defense technology has come a long way since then.
I agree with the others that there are many variables that affect this particular situation. However, I have also repeatedly heard from " the pros" that speed in the 260-270 fps range is the maximum limit for shooting fixed or replacement blade broadheads accurately. I am assuming that they arrive at this conclusion from repeated personal experience though I would offer that I, along with many others, have been able to achieve reasonably accurate groupings in excess of this " speed limit" . I believe the key to this being extensive broadhead tuning and good shooting form.
How fast is to fast?
