I used an HTC One for a few months before I went back to the iPhone. I mostly didn't like the software and the UI. The phone would lag randomly for no reason. The performance was just bad. iOS is more fluid and has a simplistic UI.
I have an HTC One and for the most part I love it. The lag you're talking about isn't an "Android" vs "iOS" thing, it has to do with poorly-coded apps. Android in general is a lot more permissive with the Google Play Store than Apple is with the App Store, so there are going to be app experiences that feel a little jankier on Android - it isn't something inherent to Android or the HTC One specifically.
The things I love about my HTC One over my old iPhone:
*Bigger screen
*Awesome phone speakers
*Google Apps are much better on Android (Gmail, Maps etc)
*My bank has NFC tap-to-pay enabled so I can just tap my phone on any Visa PayWave-enabled point-of-purchase pad (which are basically everywhere here in Canada), and the iPhone doesn't have and likely never will support NFC
*Ability for apps to work better with each other (we'll see if Apple catches up with "extensibility" in iOS 8)
*Swype and SwiftKey keyboards (supposedly coming in iOS 8)
Things that were better on iPhone:
*Camera
*App quality (a lot less of the "jankiness" I talked about earlier)
*Regular and consistent OS updates
The camera on the HTC One isn't just a minor inconvenience - it's terrible. Almost completely unusable. I've seen "sample photos" on the internet taken with an HTC One and I have absolutely no idea how other people are getting these shots, I've never been able to take a decent photo with my HTC One in any lighting or under any circumstance.
Once you get used to a big screen (4.5"+) it's really hard to go back though. The only way I'd even consider another iPhone is if they upped the screen size.
I'd say Android and iOS are about equal overall, if you're sticking to flagship phones (i.e. high-end Samsung/HTC/Sony devices) you really can't go wrong with either.