It's not that it is straight as an arrow, it's that the hitters know it's coming. I don't care if his 100 MPH fastball is straight, it's still 100 MPH. If you go to a batting cage, if given enough time, even you can learn to time a 100 MPH fastball. The problem with Parnell early in his career was he was just throwing and hadn't yet learned to pitch. Everyone could just sit on the first fastball they got because they could be 100% sure that they were going to get at least 2 per at bat. Since he has learned to throw the slider for strikes in addition to the knucklecurve that Jason Isringhausen taught him, his numbers have reflected those of a quality pitcher with good stuff and a three digit fastball.
This is where a guy like Bob Gibson shined.
He didn't need movement. His only "movement", was whether or not it went under your chin.
SO instead of hitters waiting for a FB over the plate, hitters were leery about getting too close to a 100mph FB.
"but the faster the pitch goes, the less movement. Since Gibson was not approaching triple digits, his fastball would move some."
Having sat behind the plate at Great American Ballpark last season, I can say without a doubt that Arodis Chapmans 105 MPH FB moves WAY more than Cueto's, Arroyo's, and Bailey's...all whom throw between 88 and 98.
What's gonna make Parnell a better pitcher, is to learn when NOT to throw strikes.
I recall a game last year, when he was up 0-2 in the count. Darling, and Keith were wondering why Thole wasn't calling for a pitch off or up high off the plate. Instead, he grooved one, and got beat.
He seems to get anxious, and wants to end the game. That's why he did better as a setup man. It allowed him to pitch.