The way I see it, there are 3 types of "lag".
1. Is the delayed reaction from when you touch the screen and perform an action to the time it responses. One way to see this is if your on a list, tap one of the words and scroll up and down and see how closely the word sticks to your finger.
2. Is speed, some equate slow speed to lag. This of course is how long it takes things to load, fairly easy to observe.
3. Is frame skips. This is how consistently all animations are rendured on screen.
Lag 1 really hasn't been a problem on android for a couple years, while I did notice it on my first android devises (Moto Atrix) its pretty much a non issue now, though its still slightly behind iOS, it isn't noticeable unless you have both devices side by and side and looking for it.
Lag 2 has been mostly on par with iPhone since the beginning and really depends more on your hardware. IOS had been better at hiding it by loading up a stitic image first, but both have been the same here for me.
Lag 3 is still the one that rears its ugly every once in a while and bothers me still. Now this isn't exclusive to android, all OS's including desktops will bog down and skip frames depending on what your doing, though I do notice it slightly more on my HTC one with stock lollipop that I do on my iPhone 6 plus. I do have over 200 apps on both systems though, which would no doubt contribute to this, but it seems the iPhone is better at keeping a consistent frame rate.
None of those are deal breakers at this point, android performs great while keeping a lot of versatility that iOS locks out to maintain that slight performance edge. Of course your expierence may vary depending on your hardware, android version, apps loaded, and settings.