Speeds not what you pay for

I am paying for a 25mbps line from Afrihost but I seldom get 25mbps. I mostly get between 5mbps to 15mbps. Why am I paying for a certain speed but not getting it? When you call or whatsapp support they do something and the speed is correct for 10 minutes and then goes down again. Not what I expected from them.. Tired of always contacting support.