I see a similar effect. The 'official' test of the German Federal Network Agency (which we can use to complain if we don't get the speed we pay for) says I get a download speed of 215 Mbit/s when I use Waterfox on my MacBook Pro running 10.11. This is the correct speed. It measures only 15 Mbit/s with TenFourFox and Leopard Webkit on my G4 PowerBook running 10.5. All tests with gigabit ethernet connections. When I use WiFi on the PowerBook it's considerably better and probably only limited by the speed 802.11g can do on my network.