In the Bahrain season opener Yuki Tsunoda started 14th and Nyck De Vries 18th, and neither driver made it into the points.
In FP1 in Jeddah on Friday Tsunoda was a respectable 10th fastest, three spots ahead of his team-mate.
Despite that apparent sign of improvement, Tost complained that over the winter his technical team promised that the new car would represent a significant step forward, but so far that hasn’t been realised.
"Unfortunately, I think it's currently the performance level of the car, because we expected a much better car,” he said when asked about the Bahrain race.
"We struggled in Bahrain. It looks a little bit different here. I hope that at least, we are with one car in Q3 of qualifying, but we have to do a lot of things to make the car faster. Especially on the aerodynamic side, there are different programmes going on.
"The engineers are telling me that we will make some good progress, but I don't trust them anymore. I just want to see the lap time, because this is the only thing which counts."
Asked if he really meant that he didn’t trust his engineers Tost was unrepentant.
"During the winter months they told me the car is fantastic, we've made a big progress," he said. "Then we come to Bahrain and we are nowhere. What should I say?"
Elaborating on the weaknesses of the current package he said: "Not enough downforce. Therefore the car is unstable under braking, overheating the rear tyres, washing out at the apex, bad traction, everything that you need to do a good lap time."
Tost said cost cap restrictions would not prevent the team from developing the car and even changing concept.
"The budget cap does not prevent us doing development work, this is included in our business plan," he explained. "The question is now, do we go in the right direction?
“As I mentioned before, during the winter months we saw that at least on the paper, on the computer, we made big progress in comparison to last year's car.
"But all these figures nowadays, as I mentioned before, I don't trust them anymore.
"I want to see now with the next steps, that we go in the right direction, that we can improve the performance of the car, that we are able to do this. And this is a specialist work on the floor, brake ducts, and all the other aerodynamic parts.
"I think now with all the knowledge we have after Bahrain test and after the Bahrain race, the engineers should be in a position to sort it out and to come up with new parts, which should improve the performance of the car."
He added: "We are coming up with updates to the next races, we will bring something to Melbourne and afterwards also to Baku. It's within our programme that we bring to nearly every race a small upgrade."
