I've seen a few Space Shuttle and rocket launches in-person, and it always seemed to me that the Shuttle would launch a bit slower than rockets. Today, I saw the CRS-6 launch of the Falcon 9 and its ascent seemed similarly sluggish. It wasn't until the press conference that I realized CRS-6 and Shuttle had a thing in common - live creatures on board.
I've always assumed, but never verified, that the reason for the Shuttle's relative sluggishness was to keep G forces within human tolerance. Sure, some Shuttles would launch slower than others and I'd later realize they had a particularly hefty payload on board. But in general, nearly every unmanned rocket seemed like a Ferrari next to a golf cart in comparison.
During the press conference, the SpaceX representative was asked about whether CRS-6 appeared unusually slow. The representative couldn't attest to the launch's appearance, since he was watching status screens rather than the rocket itself. But he did say that everything appeared nominal, and the payload wasn't especially large - so there's presumably no reason for this launch to have been slower than any other of its kind.
Are G force tolerances really taken into consideration when launches are planned with are living things on-board a spacecraft? Or is it just a simple matter of certain rockets having different performance capabilities, and different payloads affecting them? Or, are there other factors simply affecting the perception such as launch trajectories being more/less toward/away from the viewer?