What geological processes affect the speed of Kilauea lava flows?
The speed of the lava flow from Kilauea depends on a few things:


The amount of activity at the source of lava.  This flow you are talking about is coming from a vent that is 20 miles from the Kilauea caldera.  So, it is more like a leak, than a true eruption.  There's not a lot of force behind these types of events and thus not a lot of lava coming out.
The viscosity of the lava.  This is basalt, so it can move pretty quick, for lava.
The terrain.  The source of the lava is not near the summit, but rather far down the mountain at only 700 ft elevation.   The surrounding terrain has a very shallow slope, which doesn't allow the lava to move downhill quickly.  
The cooling rate of the lava.  It rains a lot in Pahoa.  Especially in late October.  This helps to cool and harden the lava, which slows the movement.
Barriers and surface characteristics.  There are a lot of natural contours in the land, trees, and brush in the Pahoa area.  This helps to slow down the movement.


Having grown up on the Big Island, I can say from experience that these flows that come out of side vents are often slow enough to walk right up to and play with.  They don't move fast at all.  The reason it's such a big news event is because it nearly covered a main highway, which would've cut off a lot of people from the main town, Hilo.  

The fast-moving lava flows usually come from the main caldera, where the amount of lava erupting is far greater.  Also, Kilauea is really just a pimple on the side of Mauna Loa.  So, it is a bit of a release valve for the big volcano.  Shouldn't be too long before Mauna Loa erupts again and then you will see a lot of fast moving lava from a 13,000+ ft location.