How to improve Joomla's performance
As a consultant for Joomlatools I help to improve site performance for our hosting customers. We published a short blog post summarising the way we work on this type of problem.
A lot of improvements can already be made without changing any code, using tools like PageSpeed to optimize HTML, CSS, JavaScript and images on the fly. Caching certain pages and moving to a CDN like CloudFlare can also help to solve a lot of common issues quickly.
You can read the entire post on the Joomlatools blog: How to improve Joomla’s performance