The 32-year-old, who has not raced in Formula One since a 2011 rally crash but is favourite to replace departed Brazilian Felipe Massa, was seventh fastest overall on the second day of a Pirelli tyre test at Abu Dhabi's Yas Marina circuit.
His lap of one minute 39.485 seconds compared to a 1:39.580 best by Canadian teenager Lance Stroll on Tuesday, with both on the new hypersoft tyres.
Russian Sergey Sirotkin's best of 1:39.947 on Wednesday morning was on the soft compound. Kubica was 1.3 seconds slower on that compound on Tuesday.
Williams technical director Paddy Lowe said the times could be misleading, however.
"It's a really complicated topic, performance and speed, so to read a timesheet is quite misleading. So I'm not going to talk about that. We haven't even analysed it for ourselves," he told reporters.
The Briton said Kubica had done a "fantastic" job in the cockpit.
"We wanted to see Robert as he hasn't driven a current car, or current tyres, so of course it was interesting to see how he got on with that.
"No problems. He's absolutely fine. Good driving, no complaints, no issues. All went well."
Kubica, who partially severed his right arm in the crash before the start of the 2011 season, completed 100 laps in Tuesday's session and 28 on Wednesday.
He told reporters on the first day that he felt "confident and comfortable".
Ferrari's Sebastian Vettel was top of the overall time sheets.
(Reporting by Alan Baldwin, editing by Ed Osmond)
