I use a powerline AV500 by Netgear, I get 16mb and 9mb from my internet connection.
I'm paying for 100mb down and 10mb up.
When I use my WiFi I get 25mb and 9 mb from the same connection.
What should I do to maximize what I get from my Internet connection?
That device is simply for using your house electrical wiring to extend your computer's connection to your Router, intended to make things easier than running Ethernet cable. The Internet exists from the Router out. There are possible issues with the wiring in the house in lowering your Network speed which is not the same as the Internet access speed. I'd plug a computer into the Router using an Ethernet cable to determine what your download speed is and if not getting somewhat close to what your are paying for contact your ISP.
Powerline | Networking | Home | NETGEAR
You should realistically get about 85 Mbps through a Netgear AV500. I use the same type and can stream video through three floors without delay, except one room where connection grinds to a halt. It depends on the wiring; the age of the electrics; the circuits in the house; etc.