If you're using wireless, I'd suggest you go wired. By that I mean run a Ethernet cable from your router/modem and plug it directly into your LAN port on your computer's motherboard or NIC.
With that you will probably get the fastest possible your ISP is giving you. If you're still not happy with the speed after using a wired connection, the only thing you can do as previously mentioned is to call your ISP and pay them more money for a better plan.
*Note/story - Even if you don't have extra money to give them - still give them a call. I don't know if ISPs do or can do this in Europe, but I was once on DSL at my old house with speeds up to only 1mbps. I eventually got fed up after a couple of years and called them up, only for them to then tell me that I was on an older, more expensive plan, and that I could upgrade to 6mbps for LESS than what I paid. So I did it and the speed was a bit faster, but it was DSL, so I had to deal with it until I moved to where I am now where there is an actual cable provider.
Cable aka DOCSIS based ISPs are on the very expensive side, because they are typically monopolies (atleast here in capitalist USA), but if you want a reliable and fast connection, go with cable if you can, but be prepared to pay for it.