Website performance is about two things: how fast the page loads, and how fast the code on it runs. Plenty of services will make your website load faster, from minimizers to CDNs, but making it run faster is up to you.
M. Chevalier-Boisvert, and M. Feeley. 30th European Conference on Object-Oriented Programming (ECOOP 2016), volume 56 of Leibniz International Proceedings in Informatics (LIPIcs), page 7:1--7:24. Dagstuhl, Germany, Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik, (2016)