Webgl is harder to optimize but we did our best, there should not be a big difference between full screen and in window play. Can I learn your PC specs and browser?It is ok if you don't want to give them. There is also a windows version which will be definitely more performant but you can exit the game only with ALT+F4.
You are right about the navigation but we simply did not have enough time to implement an UI that you can open whenever you want.