I have had the M9 for 2 months. Compared to the s6 is positively slow. I find that is good software but really slow. So I recommend the s6 as benchmarks prove it's superiority. Maybe Sense 7 needs a tweak or two.
Even at CPU optimization option in the Developer Options made it lag terribly. I just thought it was lag but now realise having acquired the s6 that maybe the M9 is this slow anyway. I do like Sense 7 but wish they tweaked it up a bit.