The workshop where I play with my toys is usually unheated and at this time of year can be down around 10C, I usually blast it with a LPG heater to bring it up to about 20C whilst laying up glass etc though as the wet out is sooooo much easier.

It is not so much the drying time but the sanding time if that makes sense, even after 24hours using a fast set epoxy, when sanding you will find the discs clogging with epoxy, I presume the heat generated from the sanding disc is enough to soften the green epoxy.