The game played rather nicely for me, I think the difficulty is about right - not too overwhelming, not too boring. Not sure if I reached the top speed (my high score is about 580) but I think you could slowly increase it further - IMO, it'll give a room for determined players to try and reach higher scores.
The only issue I had with the game was a lack of understanding what to do - I read the description later but having an in-game tutorial would be a good thing. Besides that, telling the player about the dash ability would be nice (I learned about it after reading the comments).
I think you could add some walls (perhaps they could appear after the player reaches some score) but that requires testing.