It is my guess that for things to work properly with large high quality samples all the GM instruments need to have the attack (first part of the sample) parts loaded into RAM. Once this is the case Kontakt or Bandstand could immediately respond to program changes. With Bandstand being something like 2GB in total it should be possible. It would still take about 30 seconds to get the rig loaded at startup.

Another workaround could be to load up only those instruments that you know you will use, e.g., all the acoustic instruments but none of the others like special effects/pads and so on. This means you need to get into the wavetable and create a new one. This can be done with Colossus and kontakt....I haven't tried this to see if it works.

The answer is blowing in the wind.