What do you mean , dont say reduce polys … 1.5 million polys for a mobile device is gonna take some strain … it has to be said. what really is bizarre, is you practically have nothing in the scene and yet so many polys … it seems to me you are missing the crutial step of having to rework models for real time display. You seem to just assume you can upload offline models and it ends there.
Models can work like this … but they always have a good chance to not work so well. Hence your problems.
It is always the same set of rules when building for weaker processing devices. you need to tailor the scene and optimize it to run at its best. That is just how it works.
I do this for clients all the time … some times a scene can be reduced by over 98% and still look the same, and yes that a 98% improvement in rendering speeds then and load time for that matter.
sketchfab is software. What can they do about the processing power of hardware? nothing. It is you who needs to adjust you workflow to try cater for the best average experience across a range of devices , make and year etc. All mobile developers have to do this.