Played in Firefox and didn't have any significant performance issues outside of some brief lag when first starting my run.
I played enough to level up three times and then stopped because I got the feeling I wasn't going to see anything new beyond that point. I found the gameplay so far to be really uninteresting, perhaps because there only seems to be one enemy type that does nothing but try to move into the player. The simple background, minimal sound design, and recycled player ship sprites for the enemies also don't help the presentation of the game. There needs to be a lot more content here to make me want to spend more than a few minutes playing.
The menus are functional but generally plain-feeling. I would have liked more information on the specs of each playable ship when selecting them, but I would also like less information on the upgrade selection screen. (For example, the first two times I leveled up, my options were four shotguns and then four snipers with the exact same text cluttering up each choice. I had to manually select each one and read the text at the bottom to figure out what the differences between each choice actually were. I think a simple descriptor such as "shot speed" or "fire rate" being present on the choices themselves with the more detailed text appearing at the bottom would be easier for the player to read.)
Overall, the game feels like a functional early tech demo of sorts. It seems like the player can become extremely powerful after playing for a long time, but the very repetitive nature of the gameplay so far doesn't hold my interest enough to make me want to reach that point.