In yesterday's thread about Aroldis Chapman reaching 100 MPH on the radar gun, it was asked what a difference 100 MPH makes compared to, say, 96 MPH. I decided to take a simple look at it, just for fun. This is in no way definitive and there are huge caveats. For instance, each park reads speed slightly differently and I have not adjusted for that. Also, movement matters just as much as speed. Fast and straight is still hittable for a big leaguer. I have not taken any of that into account.
So, with that being said, here are the numbers that I found, courtesy of Joe Lefkowitz's Pitch F/X Tool:
In Play includes HR; AvgIP is average on balls in play, including HR; IsoIP is power (slugging minus batting avg) on balls in play; Miss% is the percentage of swings that the batter failed to make contact (contact includes foul balls)
Obviously this is only one season and we are looking at the rarest of rare pitchers, so it's hard to draw anything concrete from this small set of numbers. However, it does appear that there is a distinct advantage being able to throw it 98 or faster, as you would expect. Hitters appear to be able to put the bat on the bull much easier at 97 than 98. On the other hand, when contact is made on 98, it looks like it's just as solid as at 97.
If you can rev it up to 99 or higher though, you should be in pretty good shape. Ultimately what matters most is movement and what your other pitches look like. If all you do is throw 98 MPH fastballs and the hitter knows that, most big league hitters will be able to catch up to it eventually and at that point, you've got problems.