I think you'd find that the vast majority of pop hits are written fairly quickly. Yes producing, recording, finishing a pop song, adding everything that makes a pop song.. pop... can take a long time. But the actual writing is literally just

1. Get an idea

2. put it into the standard pop song structure

Especially for someone like Pharrell who has probably written a hundred songs or so.

It's by no means _remarkably, exceptionally fast_.

Edit:

Look at the way Notch writes code. Yes okay, we could call it remarkably fast. But he does it consistently. It's not a surprise when he finishes a game during a game jam.

Just as I do not find it surprising that one of the most accomplished songwriters of his generation writes a song in an hour