>>53829206I'm using a Mew Genesect deck and yes deciding whether to pump my Fusion Energies into my Meloetta or my Mew, or whether to Boss or Elesa for more draw, can literally decide between a win or a loss. Hell, even just remembering to Path to the Peak after using all my Fusion Strike Array is something I often forget to do. Or remembering that a Lost Box may suddenly whip out a Kyogre at the end. Things like that.
Or my other deck is a Miraidon Flaafy deck and plenty of times I've accidentally pulled out the Flaafy into the active spot, not remembering that you can't Dynamotor onto the active spot. Or I put my Klefki into the active spot, forgetting that it'll stop my Miraidon but not my Flaafy. Or I forget that Flaafy has 2 retreat cost and Beach Court only affects Basic mons.
Hell, like I said earlier, I literally forgot who went first, me or my opponent. If I had more time to take note of these things I'd be fine. Or memorising how many specific energies I had left in my deck whenever I shuffled.