Consider the two following snippet:
#content %h1 Some nice headline :coffeescript $('h1').click -> alert('You clicked on the headline')
Then the performance hit is mostly gone. You still have a code smell in your views, but not a one that will cause a drastic performance loss if you don’t remove it right away.
But if you consider a situation in which you render nested partials, and each one of them has some inline coffeescript, which I myself have done unknowingly before, that will multiply the issue with the amount of rendered partials. You can of course count on rails caching to save your ass, and it will most of the times, but not always. And then you will have that one customer who waits for more than 10 seconds to get their page.