Actually, CoffeeScript does generate more efficient code than the vast majority of JavaScript developers write by hand -- that's one of the goals. There are a couple reasons why this is the case, for example:
Do you tend to write your "for" loops with an "each" ? If you're using "$.each", "_.each", or "[].forEach", your loops are running a good bit slower than they could be. This CoffeeScript:
for item in list
item.marked = true
Generates this JavaScript:
var item, _i, _len;
for (_i = 0, _len = list.length; _i < _len; _i++) {
item = list[_i];
item.marked = true;
}
... where you have a nice length-cached loop that runs about as fast as JavaScript is capable of.
Similarly, lots of JS developers avoid using prototypes because they're such a pain in the ass to type out manually -- preferring to manufacture objects via closures instead. This has a huge runtime and memory cost. CoffeeScript's "class" keyword makes it easy to work with JS prototypes effectively, without having to type out "Klass.prototype.method = function ..." all the time.