agreed with you. thats why i'm happily ditch N6p (passed to my mom) when i got myself S7e. at first thought pure nexus will be smoother (or whatever the terms is) but S7E proved me wrong.
after read his comment, just out of curiosity again (in case im wrong), took my mom n6p (which installed with bare minimum apps), clear running background apps, did the same to S7e (which have around 80+ apps),did like those youtubers, launch at same time (apps like youtube, fb, dialer, contact, etc), observed that either S7E response similar or faster (which something i expected due, after all, it have more ram and more efficient SOC compared SD810 in N6P).
(both phones in stock non-root mode, with animation etc in 0.5x for both phones, both using nova launcher with same speed setting).
anyway,in semiconductor industry, not every SoC/CPU/APU/GPU will 100% match or equal from batch to batch (with minor non-significance difference) thus perhaps that's make a different between those sets around.
multitasking wise i do admit the N6P optimize to hold more apps running in background.
anyway, each to their own. everyone have their own preferences and own life experience

Though said, it is actually not so fair to compare the Nexus 6p against the S7E due to the S7E has newer and better hardware. However, we are talking about fluidity and smoothness here. Nexus devices are known for the "vanilla android" which on paper, should be the most fluid and smoothest android without the additional add ons by different phone manufacturers. But somehow or rather I find the S7E to be as smooth as the Nexus 6P or maybe even slightly better even with a more feature laden touchwiz. I also find Nexus devices too "bare" in terms of android.