Every pre-Android 5 phone suffers from "lag" due to apps being compiled at runtime. There's no way for any pre-Android 5 phone to avoid it, because it's how the OS works. Lag within the UI itself is from poor coding, which Samsung is notorious for. Even with a considerably slower processor, the perceived performance of my 635 is comparable to my N5. The UI is extremely "smooth" on both. App launch time is comparable. The Lumia 635 was $168USD vs $350USD for the N5.