Beefing up the server cannot improve client side code. Jquery is quite slow, nice, but quite slow. Sometimes writing out those 8 lines can make your code literally 400 times faster.(source : http://vanilla-js.com/) So yes if performance is a concern, you might want to look at writing it yourself.

Very good point, I'd not really considered the lower end spec'd machines, was more thinking overall page load times and the likes. Thanks :)

So if I buy a beefier server, my users with a netbook (ca. 2007) will feel a responsive[1] website?

[1] Responsive as in "Mozilla Firefox is not responding", not as in "Responsive Design".

Good point, not well made. Thanks for the response.