Hi, so , we could make a top 3 list of things that create troubles with LDhilbert + DP3rd : scene geometry complexity .
Not just polycount , e.g. even a 50k quads scene with things like niches , long curve corridors , porches , will make complicate lighting paths.2nd : materials
matte translucent , perfect matte , strong bump (i think)1st : lighting : sun or many small, very bright artificial lights.
Lighting seems the real problem. the other 2 cause slowdowns which are ..absolutely normal
this is an interesting comparison : the bright yellow spot on top is sunlight ,
the remaining light is rectangular mesh emitters.
so 'dumb' samplers solve easy light fast ,
metropolis is slower but can solve difficult ligh sources.
What makes sun a difficult lamp is the 'penumbra' area : where a dumb (random) sampler sees only few rays but this rays have still an important contribution.
Metropolis takes that into account , but on the other hand takes many samples to converge (even for simple lighting)
Might be useful to distinguish that noise (in the hilbert test) from 'fireflies' caused by reflective caustic , which are smaller ,less frequent and generally brighter
(one way to distinguish them is using the maxwhite tonemapper : with fireflies the rest of the pic will be pitch black , if it's just noise you'll still see some of the remaining light)