(Suggestion) asynchronous games
this game is really slow and it takes a long time to play many duels.
I suggest having an asynchronous mode where you can play hundreds of players at once. Basically you would have a list of games and in the list each game would be flagged for if there is an action for you to take, or when it is your turn. If theres an action, you load in and do the action (or your turn, if it is your turn) then the game is saved and flagged for your opponent when it is their turn or an action they need to take to continue. You could also take as long as you want to do your turn, plenty of time to sift through the discard piles to remember what was going on in that game, if you need to.
and of course, it could be totally optional. I am not suggesting replacing the normal way to duel. You could also conventional duel if there are no games for you to update.
or, another good option would be to run 2-3 duels at once and just be able to flip between them.
Players are too slow for duelling the conventional way to be fun. The conventional way makes sense for casual play in person, but for an online game there is no human connection, you cant shake their hand or say “wow, well played” or anything. Having to just wordlessly emotionlessly wait on each other doesnt feel good honestly.