McLaren had its strongest qualifying performance of the season in Barcelona with Lando Norris taking a surprise third on the grid, while Piastri earned 10th before gaining a spot from a penalty for Pierre Gasly.
The drivers cautioned before the race that conditions in qualifying had flattered the car.
However, the race proved to be even tougher than expected. Piastri slipped back to 13th, while Norris could only recover to 17th after first lap contact with Lewis Hamilton led to an early pitstop and ruined his day.
"Yeah, it was just a long afternoon,” Piastri said. “I just didn't have the pace to really do anything.
“Pretty poor first lap for myself as well, which didn't help things, but I don't think we really had the pace to stay in the points anyways.
“We'll have a look at why we struggled so much today compared to yesterday. I think we've got some good ideas already. But yeah, disappointing afternoon."
Piastri stressed that cool conditions on Saturday had favoured the car over one lap on soft tyres.
"I think yesterday [Saturday] was obviously not expected,” he said. “I don't think we're in normal circumstances able to put the car that high up on the grid.
"Also, today [Sunday] was maybe a bit more back to reality. Probably a more difficult day than what we would have hoped, even considering.
“We seem to switch on the tyres very well when it's cold, when it's difficult conditions. Monaco was the same story as yesterday on the inters. So I need to try and work on that with the team, I think, and see what we can do to make our lives easier on Sundays.
“I don't have that many answers at the moment. But others seem to just be able to push more when they wanted to and find an extra gear, and we were kind of flat after that. So I think something to do with that."
Like fellow rookies Nyck de Vries and Logan Sargeant, Piastri had to get through a difficult run of street and temporary circuits prior to Spain.
However he downplayed the suggestion that the greater margin provided by the Barcelona track had been a benefit.
"Not so much,” he said. “I think the risk of damaging the car here is lower, but I guess the lap time penalty for not finding the limit is probably higher than those circuits. Because everyone's got a lot of laps especially around here you can really find the limit without too much risk.
“So everybody is just really pushing to the max. From that point of view, I guess it's a different risk. I definitely felt a little bit more comfortable here than I did in Monaco, especially in quali.
“The last few weekends it just sort of comes alive in quali for me. So I still want to work on getting up to speed a bit earlier in the weekend. But yeah, looking forward to another new track [Canada] next time out. It's a tricky one as well.
“So that'll be fun to learn a new track, but results-wise coming to a more familiar track didn't really make much difference."
