The video from the official Formula 1 youtube channel show´s the big progress Williams made from last year.
The rear is much more stable, especially under acceleration out of tight corners.
If you compare the lap times with Mercedes during qualifying in Bahrain over the past 5 years, it´s obvious that the FW43 is the first car since 2017 that is an improvment over its predecessor.
2016 2017 2018 2019 2020
Mercedes 1:29.493 (Q3) 1:30.814 (Q3) 1:28.124 (Q3) 1:28.190 (Q3) 1:27.264 (Q3)
Williams + 1.66 (Q3) + 0.56 (Q3) + 3.29 (Q1) + 3.57 (Q1) + 2.03 (Q1)
In 2018 and 2019 they massively lost ground by changing the concept of the car completely each time. Now they seem to have a stable platform they can build on.