Robert Kubica says Monaco is highlighting the shortcomings of Williams's Formula 1 car "even more" and making it tougher to push harder.
Kubica was four seconds off the pace in Thursday practice, albeit less than a tenth slower than team-mate George Russell.
The grand prix winner, returning to the scene of two podium finishes in the first part of his F1 career, said the main positive was that his "opening laps were not too bad" despite not driving the circuit for nine years.
"With the lack of grip we have, and struggling with general downforce and general grip of the car, Monaco is not the easiest place to drive," said Kubica.
"Unfortunately the feeling is not great, which was predicted. You hope for a good surprise, but there are no miracles.
"When you lack general grip, in Monaco it highlights it even more, and this makes it even more difficult to push the car and to have enough confidence to be able to squeeze something from driving."
Russell said Williams was "probably slower than we wanted, and maybe what we expected" after appearing to make a step closer to the rest of the field at the previous race in Spain.
However, like Kubica the reigning Formula 2 champion suggested Williams's fundamental limitations were making this event harder.
"In Monaco you need the confidence in the car and a consistent car and at the moment we're lacking that," he said. "I'm not too surprised."
Russell has targeted making "the best of a bad situation" and to "continue" learning through his first Monaco GP.
He hopes that the race proves to be a one-off and that Williams's pace at Barcelona is more indicative of progress being made.
"Monaco is a very specific circuit and if we took a step backwards this weekend we won't be too disappointed because it's not too relevant for the rest of the year," said Russell.
"We're finding it a bit of a challenge at the moment.
"The pace at Barcelona was much better on a representative circuit so we won't take too much away from this."