`GPU mode + max ray intensity 20 + mesh light on the ceiling at 50
OK, I can disable speculars and the fire flies are gone. But maybe it’s possible to avoid the fire flies by an other way.
Hi Micha, are you using default settings? progressive or bucket mode? the lights I see are real lights or just emissive?
The lights are mesh lights and I use the progressive GPU mode.
Big surprise - I found if I remove the mesh lights from the scene and use rectangular lights instead, I get no fire flies, less noise and the render time is down from 4min to 2min. I used the mesh light at the small spot at the foreground too and replaced it by emitters → render time 1min22. Looks like mesh lights should be avoided if possible. Is the mesh count for mesh lights important?
procedural shapes are generally highly optimisable (square, sphere, and so on), while mesh lights need a whole lot more effort to be calculated (there is no way to get to know the topology of the source. that it’s a flat plane is something only the user knows for sure, and V-Ray can’t assume.).
To answer your question: yes, keep using emissive shaders and mesh lights as sparingly as possible, and use standard lights whenever there’s a chance.
Particularly with adaptive lights, modelling f.e. a led strip with a gazzillion square area lights is going to converge a whole lot sooner than doing the same with emissive shaders or mesh lights.
@_Lele Thank you, this helps me better to understand the usage of the mesh lights. 
Hi MIcha,
Big surprise - I found if I remove the mesh lights from the scene and use rectangular lights instead, I get no fire flies, less noise and the render time is down from 4min to 2min. I used the mesh light at the small spot at the foreground too and replaced it by emitters → render time 1min22. Looks like mesh lights should be avoided if possible. Is the mesh count for mesh lights important?
This is just about right.
I would strongly advise you to use Area lights every time you can get away with it.