Should I play fast games or slow games to learn more?

"..., you have to make a decision: have tons of fun playing blitz (without learning much), or be serious and play with longer time controls so you can actually think.
One isn't better than another. Having fun playing bullet is great stuff, while 3-0 and 5-0 are also ways to get your pulse pounding and blood pressure leaping off the charts. But will you become a good player? Most likely not.
Of course, you can do both (long and fast games), ..." - IM Jeremy Silman (June 9, 2016)https://www.chess.com/article/view/longer-time-controls-are-more-instructive

both of course. in another thread GM Eric Hansen said Blitz helped him become a better player. 10 15 minute games should be good to improve

Like previous poster said, if trying to learn always run your game through the analyser before starting another and work out what went wrong. Many bleat if you do not accept a rematch but just ignore them. Better to look at the game, even if it a 2 minute game.

what happened when you tried?

"..., you have to make a decision: have tons of fun playing blitz (without learning much), or be serious and play with longer time controls so you can actually think.
One isn't better than another. Having fun playing bullet is great stuff, while 3-0 and 5-0 are also ways to get your pulse pounding and blood pressure leaping off the charts. But will you become a good player? Most likely not.
Of course, you can do both (long and fast games), ..." - IM Jeremy Silman (June 9, 2016)https://www.chess.com/article/view/longer-time-controls-are-more-instructive

Like previous poster said, if trying to learn always run your game through the analyser before starting another and work out what went wrong. Many bleat if you do not accept a rematch but just ignore them. Better to look at the game, even if it a 2 minute game.
Or you play Daily, and you can rematch, as you have several days to make a move.