One of our users is employing mylar space blankets to speed the initial heat-up process, particularly where heat-absorbing materials like concrete are present.
The idea is that heavy heat absorbers are covered during the initial heat-up so that all the heat goes into the air and other items in the room. Then, once the room is up to temperature and the heaters are cycling on and off, remove the space blanket and subject the previously-covered space to full, lethal temperature at a time when the heaters have excess capacity to heat the covered area.
Space blankets are super-cheap, and if they save 30 minutes or an hour during heat-up, they easily pay for themselves. This seems like it ought to work, but we don’t have any field data. If you try this idea, please let us know about your experience.