A simple idea came to my mind. It would be just 1 bit of data per every card. A “favorite” flag, to greatly speed up deck building / deck switching.
If we could “star” select cards we like and then filter to show just the favorities (similarly to how we can now filter cards by whether they’re mastered or not), it would make switching or rebuilding decks almost instant.
Perhaps it would be easier to implement than saving multiple decks per planeswalker. And it could be even more useful in some cases.