I've only had my Samsung Galaxy S5 for about 2 or 3 months, and it is getting somewhat laggy. This seems to be coupled with a severe overheating problem, where the phone becomes very uncomfortable to touch and the battery mysteriously drains very quickly (to the point where if I'm charging via USB 2.0 from a computer USB jack, the battery level continues to drop, even when plugged in).
The lag sort of manifests itself as lagging when you're typing (no words will show up on the screen and the haptic feedback won't work) and just generally slow behavior. It really sucks because I left my Droid 4 primarily because it had gotten so slow that it was impossible to use. It also had terrible battery life so Samsung's battery drain issue is also disappointing!