http://youmightnotneedjquery.com/ -- I'll just leave this here.
Especially when the 8-line version might need testing in every possible browser out there for quirks and bugs. jQuery's already done the legwork there
Edit:
youmightnotnotneedjquery.com linked in a sibling of this comment explains it better in this googledoc
https://docs.google.com/document/d/1LPaPA30bLUB_publLIMF0Rlh...
Specifically in cases where other optimizations (caching, beefier servers, faster SQL, SSDs vs HDDs, etc) didn't help more?
[1] Responsive as in "Mozilla Firefox is not responding", not as in "Responsive Design".