Thanks for the feedback and I'm glad you had fun playing! Sadly, the game gets a bit janky when playing with 4 players, and I'm pretty sure the reason why the boss health feels like too much is because about half of the attacks disappear with the lag. We haven't gotten that many comments about the health from players who were playing in groups of 3 or less (in fact, quite the contrary, some people complain about how the last fight lasts too little). But you're right, it's definetly an issue we have to tackle.
I can definetly see how the stat boosters can feel unimpactful at times. They were mostly added as a bonus (since they were pretty quick to develop) and as a fun way of making players say "wow, look at how absurdly massive my weapon got lol" or "I die in one hit but I can just dash across the entire arena wooo". They were made rarer for this reason, but they sometimes still appear more than they should, and that's one of the things we'll probably be tweaking first whenever the jam ends.
And about the sword speed, we straight up didn't notice since it's only a problem with swords, so thank you for telling us!