it would seem like the highlight at the bottom of the glass sphere has moved to the top in Luxrender
I'm surprised how different all 3 look as far as specular highlights go
Indigo was much faster under artifical lights than in the Sun compared to Thea and Lux. That whole lit area under the water was rendered in 1 big sweep pretty much, instead of little "dots" in the other 2. The light is a 10x10x5cm mesh light
Thea, Indigo, and Lux, in that order
Found it. First I was seriously impressed with the Indigo result compared to Lux and Thea, but have a look at the differences. Lux and Thea have different noise but similar light distribution while Indigo is completely different. Just look at the highlight on the right column in the back or the brightness of the big spot under the glass sphere...
I then had a closer look at the light source and bingo: The light source a box that is open at the bottom, and the normals of the faces point outwards, i.e. to the sides and up. Lux area lights emit only in normal direction, most of the light got trapped between the roof and the light. Only the light that was actually sent to the pool was either indirect light reflected by the roof and the columns or the small amount of direct light coming from the thin side faces. No wonder that it took ages to converge.
Thea obviously behaves the same, but Indigo seems to emit light from both sides of a face. To reproduce the Indigo rendering, I inverted the normals of the light and duplicated the side faces and inverted them again, i.e. the side polygons emit now in both directions and the big top face emits down.
After baking the scene for 30min I get this result, it's not fully converged yet, but not too bad either:
I hope this helps.