Much much slower, you tend to play differently. On one hand as you can't put down suppressing fire you are much more sneaky and on the other you play a bit more aggressive as you generally need to get closer to make sure those single shots count. It's a very different style of play and can be a lot of fun but also quite frustrating. I've never played a pump only game so I usually take mine out against people shooting 'normal' markers.
Think "pump action shotgun". The marker still uses an air bottle and fires balls at the same speed/distance etc - you DO NOT pump the air into the marker!
Pumps were the original style of markers (after small pistols) that were used in the beginning of the sport, and are used by a lot of people for nostalgic reasons, and because there is a different set of skills in using one, as described above. I have one, which I use when playing with friends at rental days, as mowing downs people playing for the first time with a tournament gun is a tad unsporting