McLaren had their best qualifying of the season in Barcelona as Lando Norris secured a shock third place on the grid, while Piastri also reached Q3 and took 10th.
But McLaren’s promising one-lap pace failed to translate into a strong result, with the team ultimately leaving Spain empty-handed.
Piastri slipped to 13th at the chequered flag and Norris finished a lowly 17th after first-lap contact with Lewis Hamilton ruined his race.
“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.”
The Australian added: “I think [qualifying] was a very good performance from the team,” he said. “I think [the race] was probably more back to reality, I would say.
“We just need to understand why we struggle on Sundays and when the track’s a bit hotter and stuff like that. We’ll go and have a look.”
McLaren head into next weekend’s Canadian Grand Prix occupying sixth place in the constructors’ championship, 23 points behind fifth-placed Alpine.