I'm a fairly new player, so take this with a grain of spice, but it seems to me that the best remedy for slow-play is to actively cultivate a culture of "fly casual".
I think this culture is mostly there already, but it's difficult to promote that attitude at the professional-level competitions. Afterall, most competitors at the venue - even the ones very interested in good sportsmanship - are not there just for fun. They are there to win (and also have fun, but that may not be the core of the experience at that level of play - which I think is also fine. Games don't always have to be about pure fun).
'Fly casual' doesn't necessarily apply when I really do want every edge I can get to defeat my opponent.
I am baffled by your concept that players have to choose between playing to win and having fun. I am having the most fun when I am playing to win all out against an opponent that is doing the same, regardless of the result, because I will either be rewarded for my skill or I will learn from my loss and become a better player.
And "Fly Casual" doesn't stop the issue at hand anyway, which is unintentional slow play which happens to affect the game, and how to prevent it.