As soon as water is visible in the screen, everything becomes extremely slow. I tried to put off all the options, but couldn't get any better.
And I am stuck with a glitch just after removing the fog in the second level : the two small steps uphill that require simple jumps can't be passed, since the character remains in a "jumping" attitude and doesn't actually land on the first step.
I am using Chrome on a 3 year-old laptop (only 4g RAM), if it is of any relevance.