Most likely cause of "slow" overall speed is the "ambitious" claims of your ISP: "...up to 20Mb/s" meaning more like 10Mb/s normally... your WiFi link is capable of much higher speed but the bottleneck overall is the ISP link.
A Business ISP account should have lower max. speed but low contention as well, so normal speed should be close to max and guaranteed as well.. which is why business ISP accounts cost more...$$$