My first Android phone is also HTC desire HD I can still remember how lag the phone was then, followed by Samsung Galaxy S3 then Sony Xperia Z2. Soon Samsung Galaxy S7.
My Android experience improve from Galaxy S3 onwards with Android ICS then JB.
Technology has improved leaps and bounds, single core to Octa core, RAM from 768MB to 4GB, SD card from 32GB to 200GB,screen size from 4.3 to 5.1 inch. Battery capacity from 1450 to 3000mah, Android Froyo to Marshmallow.
It's really software, not so much on hardware that affects the lag.
Having use s6 and note5 for sometime, they are very smooth, seldom lag. Same for HTC phones in general even when they use the mediatek SOC, no lag at all as sense ui is very optimised.
Lg g4 sometimes do lag and homepage refresh quite often. Oneplus2 also lags sometimes despite having s810 and 4gb ram. Nexus 5x is the worst of all in last year phones, though stock android and almost same hardware as g4 and motox style, it lags very often....
So my conclusion is software is key. It already shows that iOS only need sufficient hardware and it can run smoothly.
Stock marshmallow in my opinion is the worst android version so far. Lags like hell and camera app freezes frequently which makes me lose many important shots many times.
Samsung touchwiz in general is very good and since s6 had become better, almost as smooth as sense ui.
And 64gb version.
