Sense and Touchwiz fragment the UI and generally lag more compared to stock Android UI. On top of that, it takes them ages to port the UI skins to the latest Android version which delays the upgrades by several months
It's not the UI which is fairly high-level code. See the Android source code for the default Launcher application. What takes time is porting a new Android distribution and kernel to that custom hardware of yours, test all of your hardware-facing kernel code again completely, and make sure your kernel ABI can still talk sensibly with your userspace counterparts since things and APIs might have changed since the last Android version.