My website has been becoming slow somewhere during development, according to visitors who emailed me, what speed optimizations do you recommend to priotize? Caching? CDN? Any input is very appriciated! The server is running Ubuntu 14.10 with Apache vlatest and PHP 7.
Whenever you need to figure out what to optimize and what the priorities are, I suggest using GTMetrix or similar analysis tool (Pingdom, Google Pagespeed Insights) to get a report of what could be slowing your site down as well as suggestions on what to fix.
FWIW, GTMetrix gives your site very, very good scores. You may be worried about nothing.