Congrats on finishing the jam!
I enjoyed this game, and certainly more than Jump King - because I'm good at this one. The little animation on the sword were a nice touch and I really like the direction indicator, it helped a lot. I absolutely got some Jump King vibes when falling, or missing a jump. I always felt like the missed jump was fair because I could view the approximate arc of the sword thanks to the indicator and the terrain.
I didn't really run into any performance issues on Web (Firefox) however, the audio for the game is very very quiet. I needed to turn it up quite a bit. The audio itself matched the theme of the game well.
I think that that the main way to make it more difficult is the level design. For a Jump King like game it really needs to be fair. I felt like there could've been more recovery zones, so it wouldn't feel punishing when you fell, and include more level gimmicks (ex. the rebound stone was nice). Because you are a sword you could also cut through bats/enemies in the air while moving around or slice ropes that change the level layout and make things more or less difficult.
Great job!