I cannot understand why lately internet browsing is very fast (as it was before) but online gaming (namely FIFA ONLINE 2) has become slow to load but while playing the football match it is fast (thus not the graphics card or RAM ecc.). Any suggestions?

Routing issues with your ISP.
Didn't I already advise you to contact them directly?

BUt what am I going to tell them and if they come and check they will find the Internet running very fast!

Explain what you have posted here, it's that simple.
They don't need to visit your house at all, but look at your settings and network routing on their end..

If you are using a router this is a common problem...

With PS3 and Xbox 360 there is something you can check called NAT settings.

You need an OPEN NAT, which means you have more ports open to allow your console/PC to transmit on specific ports.

Easiest way to resolve the problem (again if you have a router or a gateway) then login to the router/gateway and set the device your using to DMZ... you'll need to know the IP of your PC/console on the network... run an ipconfig on your PC and set that IP as your DMZ on your router/gateway.

I appreciate a lot your help but I have a castlenet router and I don't know how to set it to DMZ. If fact when I went to the DMZ section and entered my IP there was this message: The router cannot be configured as the DMZ. Thanks for your help still

My Ip ends in .1 and the above appeared; then I typed .10 and it worked; could it be that I have done it right? Thanks again

Ok no problem. I found out that it didn't end in .1 but .14 (I used the cmd to find out). Thanks for your help. I think it helps.