What did happen with Williams in last 2-3 races? (Image from today's FP2)

Where does Williams performance come from? I understand, upgrades, but this good? I remember their car had flat bottom in miami, there was even a meme comparing it to Red Bull. What is happening?

Update 4 Hours later: As TesterJ pointed out, Williams are really fast in high-speed sectors. Here is a graphic comparing Williams and Red Bull, I stole from Formula 1 Tech Talk

Indeed, they are pretty good