Williams F1 team's pace deficit is 'big' - Valtteri Bottas

After a miserable season in 2013 where the team finished ninth in the constructors' championship, Williams turned its fortunes around last year, ending the season third in the standings and second fastest in terms of pace behind Mercedes.

But following Ferrari's strong start to the 2015 season, which included Sebastian Vettel's shock win in the Malaysian Grand Prix, Williams has lost its position as Mercedes' closest challenger.

While Bottas finished fifth in Malaysia, three seconds clear of team-mate Felipe Massa, the Finn was 1m10s adrift of race-winner Vettel.

Some of that time was lost to a poor start that dropped him from eighth to 14th and mistakes during pitstops, but Bottas conceded Williams lacks speed on race day.

"We maximised the result but we are missing race pace - and quite a big amount," said Bottas.

"With the ideal race, starting at the front, the gap wouldn't be that big but it is too big, much bigger than we expected.

"It shows we have a lot of work to do. There is a lot more potential, we just need to get more performance because it is possible others made huge steps."

Bottas pinpointed slow-speed corners as an area where Williams needs to improve.

"In traffic, we seem to struggle with traction a bit in slow-speed corners compared to Toro Rosso and Red Bull," said Bottas.

"Toro Rosso seem to be much stronger where we are weak, which is slow-speed, mid to exit.

"We have some updates coming and all we can do now is learn from anything we can."