Try this site, (http://www.speedtest.net) and use the world map to select Portugal if you're currently in the Holland or Holland if you're currently in Portugal. The results there are listed in Mbps (megabits), and selecting the alternate location will allow you to test available bandwidth and latency over transcontinental links. Don't worry if your speeds don't match your ISP's advertised speed since those are usually burst not sustained.
Here's my test results.
Now for the reason you're only 1.8 Kbps is due to your current conditions. the Slingplayer has a two-way communication with your Slingbox. Its algorhythm will adjust the encoding to give you the best picture with the least amount of bandwidth possible under your current network and hardware conditions. If you'd like to see your current statistics then check below.
Hold down [Alt]&[Shift]+[i] for the Slingplayer Desktop
For the Web Slingplayer, hold down [Ctrl] and click on the picture.
If you still believe that you should get better, then you can go into your Slingplayer Options (Alt+O) and manually tweak the settings in the Encoding Tab to adjust the stream parameters.
Thank you for your answer! The site you sugested me gives me 25 Mb download with conection through the router or direct! So the problem does not seem to be the bandwidth that I'm receiving, but maybe something that I have to change in my router... because it is really a matter of the type of connection! With exactly the same manual settings on the encoding tab, with direct connection it is fine (3600 Kbps), and when it goes through the router 1800 Kbps!! Is there something general that I have to activate/deactivate on the router?
In general, I can recommend that you make sure you have port forwarding rules for both TCP and UDP. If your router supports Quality of Service (QoS) then you can grant priority to your Slingbox. Something else you wanna check is if you're using the most up-to-date firmware.
Other than that, you might consider doing a google search to look for tweaks to improve latency for your router