Williams, fifth last season but without a win since 2012, use the same engine as champions Mercedes but lost their way on the aerodynamic side with a problem they have been unable to do much about.
At the same time, Ferrari-powered Sauber – last overall in 2017 with a year-old engine – have become much more competitive with an up to date power unit and have lifted themselves to eighth with 36 points.
“It’s been tough to see some of our competitors early on, like Sauber for example who were neck and neck with us at the beginning, make big steps in the right direction, compared to us,” said Stroll.
The Canadian, whose current team mate is Russian Sergey Sirotkin, said he had still matured as a driver in his second full season and “absorbed a lot of information”.
“As a driver, you have to accept that some years are more challenging than others and that’s part of the game,” he added.
Williams have so far confirmed only one driver for 2019, Britain’s Formula Two leader George Russell.