Why do house geckos take so long to react?
This is entirely behavioral and is quite widespread. 

If they ran away everything something large passed by they would waste a lot of energy, many animals wait first to see if you are just moving through, and thus can be ignored, OR if you are going to hang around and thus are a potential threat (and in the case of camouflaged organisms have a much higher chance of spotting them). 

It is seen in many animals gazelle are known for it for instance.  This paper on the behavior in other lizards should help with understanding the evolutionary pressures behind it.