Usually the hardness of the material is considered primary for nozzles used with abrasive filaments. Has friction between between nozzle material and abrasive filaments been considered? It seems that friction as well as hardness would play a roll in how long the nozzle lasts. Perhaps, this is considered with ruby tipped nozzles.
Yes, Friction between nozzles and the material plays a part in the printing process. Eventually, the nozzle diameter will widen if the printer is using an abrasive filament that is using things like carbon fibre or iron. The ruby nozzle is essentially a catch-all that won't wear out no matter the filament you buy. However, the steel nozzle is good enough when it comes to most abrasive filaments; and it has the added bonus of insulating the molten filament from the outside air much better than the much more thermally conductive brass nozzles.
Friction is known to differ and it has been considered at least in some products: here Micro-Swiss sells a plated nozzle with reduced friction and increased hardness.
It is not clear from any known measurement whether the friction plays any role in the print performances, since so far lower friction coatings (Nickel) are also harder, therefore you never know whether the increased lifespan is due to the reduced friction or the harder coating.
Also, I am not aware of any test where a lower friction nozzle results in higher flow rate and print speed. I am only aware of a diamond-filled PLA which claims higher printing speed, confirmed by CNC kitchen (last graph).