That and SWA is always in a hurry!
The larger jets do actually get fairly slow depending on their weight. At least compaired to the twin turboprops I'm flying.
That said, they always look so slow just becuase they are so big... optical illusion and all that. Your mind decides the larger jets are a lot closer and thus their relative movement seems slower.
On the beech 1900 the approach speed is 130-140 kts. However, on a 767 in normal operations the approach speeds can be anywhere from 130 to 110 or 115 (i'm just picking out numbers here).
We have a ref speed (both in the 1900 and the heavies) which is our ideal threshold crossing speed for our weight. In the 1900 we have a published approach speed that we use regardless and then slow to ref as we reach the threshold. Jets, however are much "slicker" and harder to slow down so they use ref speed plus 5 kts for their approaches. So if a jet has a ref of 115 they will be shooting their approach at 120 whereas we will be around 130.
So the speeds of the jets actually can vary by a fair amount depending on weight and other factors.