Senoufou wrote: They take ages to heat up...

The early ones certainly did, but modern compact fluorescents (CF) are much quicker in my experience. The early ones - sometimes distributed free here the UK - gave them a bad name.

BobL wrote: this "waste" heat makes a small contribution ... do they take account of this

No. But if they're ceiling lights they won't warm the room air much. So put them on the floor.