Actually the metro is horrible in the summer, as if there was no ventilation.
Which line exactly? Visited Barcelona in august - don't remember ventilation issues.
Yeah, I was there in the height of the heatwave 3 years ago. Walking and transit was fine.
Old lines mostly, like the 1 (red)
Some of the lines are extremely uncomfortable in summer afternoon (was there July 2019 and there was a heat wave for a week with temps of over 100 F), but that was record setting at the time and I was only a tourist.
I used mostly red this year, sometimes the green one too. Much worse than Valencia for example.
The Barcelona Metro is fine. I used it all summer with no issue, trains come ever 4 minutes and the each car is ventilated and air-conditioned