According to the stats gathered by HTTP Archive, the average response size (i.e. after compression) of a JavaScript file is 15kB: http://httparchive.org/interesting.php#responsesizes

This will make JavaScript parsing faster for the vast majority of sites, and for the majority of users.

If you have a single JavaScript file that is 1MB after gzipping, you really should think about modularising your code and lazy-loading the bits that aren't immediately required. See http://ajaxpatterns.org/On-Demand_Javascript