@FurryNutz My PC has a Gigabit integrated LAN adapter (already posted here that I get 1Gbps connection between router and PC) and I also don't have a modem from my ISP. Please read my posts carefully. With direct PC connection I get 100mbps and with that I get ~97mbps on speedtest.net, so there's absolutely no problem neither with my PC nor my ISP connection.
I think the problem is with the H/W used in routers, maybe the ports are not properly powered, not powered at all, not filtered or not amplified properly.