Like I said, it's no problem in windows XP which is on another laptop or on my girlfriends laptop running vista. I'm using the proprietary drivers from broadcom which were automatically activated and downloaded with ubuntu. I had 9.04 earlier but upgraded and the problem did I have with both versions. The problem is always there: if the computer were newly rebooted or out of hibernate mode. Dunno what it could be.
So it cannot be my connection. The problem lies on this computer. I didn't have the problem when I was running windows 7 on it either so it has to be linux. I always use firefox so either it's the OS, its driver or it lies on the linux edition of firefox. I wanted to test chrome but there seems to be no linux version yet or maybe there is a beta somewhere.
[EDIT] The download speed is fast like always when it comes to downloaded files in the browser or bittorrent. It's the browsing that is rather slow.