There are many reasons for a website to run slow one of them is heavy content i.e. the website content received by client (browsers) is larger in size and on slower connections this could be a nightmare. Now using Mod_Deflate you can compress the size of files before delivering it to clients. Mod_Deflate is based on a class of data compression algorithms called Deflate that allows the exact original data to be constructed from the compressed data. Mod_deflate drastically reduces/compresses the files which are sent to Client (you can imagine the speed to load 26KB file instead of 120KB of your website).