php web system server infrastructure

Question

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