Is it external or inline js? In general it should get a LOT faster but that’s assuming the UI didn’t depend on the js. The easiest way to show it is to run a test with video capture enabled of before and after and then create a side-by-side video of it (go to the test history, check the compare boxes for the tests you want to compare and hit the button).
Before doing the work an easy way to see what the payoff will look like would be to block the js files in question which will prevent them from loading and should give you a good idea.
To do a comparison you just need the test ID’s for the individual runs you want to compare. The filmstrip view has a “advanced customization options” link above the waterfall that explains the URL structure (basically just a comma-separated list of tests).
The UI-way to do the comparison is to go into the test history and check the comparison box on the different tests you want to compare.
From the filmstrip comparison page there’s a “create video” button that creates the brady-bunch videos.