The Briton had his worst qualifying session of the year, although he remains to be beaten on Saturday by a team-mate in Formula 1.
At the Silverstone double-header, Russell revealed Williams was sacrificing qualifying speed to improve race pace, but that one-lap speed seemingly disappeared in Barcelona as only his Williams team-mate Nicholas Latifi and Alfa Romeo driver Antonio Giovinazzi were slower on the day.
"I was happy with our performance step but nevertheless, we need to understand where our Saturday pace has gone," said Russell.
"With these hot track conditions we are struggling to get the tyres in the right window. It’s a bit strange as on paper, this circuit should suit us better than the last two weekends at Silverstone.
"We need to look deep into it, understand where our good pace has gone, and rectify it."
Latifi, who was almost half a second slower than Russell during qualifying, admitted his best time still wasn't to his satisfaction, the Canadian starting from the back row for the third time in his short F1 career.
"On my final lap, there was probably another two-and-a-half tenths to find, but it obviously wouldn’t have changed my position," the Canadian rookie admitted.
"We didn’t have the potential to get into Q2 as we have shown at previous races with the hot track conditions not suiting our car as much. "
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