What is the mechanism by which they take so long to get going? Is it
some mercury vapour thing whereby the mercury takes ages to go into
gas phase? As you can tell, I have no idea what's going on inside
these bulbs. I thought the pressure was so low that the mercury was
always gaseous even at room temperature, but I'm not sure. That's
pretty embarrassing.



The higher power/more compact CFLs use a mercury amalgam instead of pure
mercury in an effort to control vapor pressure over a wide range of tube
temperature. The result is that when the lamp is cold, very little mercury
is vaporized and very little light is produced. For some reason some lamps
are considerably worse with this than others, to the point of being useless
for most indoor lighting, and yes, they do tend to get worse with age. I
have some excellent CFLs, but I've also had some junky ones.