infrastructure web server system in php

I need a help with respect to the web server infrastructure that I set up in the company's office, come on. The system was developed in php, I installed linux slackware 14, apache 2.4, php 5.6 and mysql 5.7, I hired the internet 60 mega download and 6 net upload, I made the port redirection, because the net blocks port 80 of physical person plans. The computer is a core 2 duo with 3 gigabytes of memory. The question is: the system accessed from outside is slow to load some pages, those that have few data loads fast, others take time. I would like to know what I should do to improve system performance.