I don't know what other devices you've used but Samsung uses its own keyboard on some (maybe all? I've only used a few) of their phones. Its not the fastest thing out there. Switching that out with the Google one can make a huge difference in perception of the phone's performance.

I've used HTC, Samsung, and currently use a Nexus5 full time. Whenever I have to use friends iPhones now they feel really dated compared to the Nexus 5. Android really turned a corner not long ago with 4.x and devices like the Nexus 5.