It is important that your web site display its pages quickly for your readers but also for the search engine that gives poor notation to slow web sites.
The first thing to do is to measure the display speed of your pages. Using GTmetrix and Google Page Speed Insight (there are others) will measure the display speed and also give you hints on how to correct them. Yslow (Yahoo tools that can be integrated to Firbug plugin in Firefox) is also really interesting.
For more information switch to the French version, most of the given links are in English. Request for a translation in the comments if needed.
Happy perfing !