This doesn't make any sense from an electronics viewpoint. CdS cells operate on light, not heat (ok, at the molecular level, the energy in the photons is dissipated as heat, and all resistors change value according to temperature, but I don't think your meter has a temperature correction circuit!). Exposing them to bright light to speed up response can't possibly work. The cell is the only light sensitive part in the meter, and if you could add light to make it "faster", that would also affect the reading since you would be measuring the sum of the two lights. Small variations in readings at low light levels could be caused by flare from nearby sources and a host of other things. I think a controlled experiment is in order...