[other] Wireless network gets slow after a couple minutes after login

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http://ubuntuforums.org – I just installed Ubuntu 13.04 on my MacBook air, and have this really weird problem. When I first login and open my browser it's incredibly fast. I use http://www.tweakers.net as a benchmark, and that page loads instantly. The google frontpage also loads instantly. However, after a couple of minutes it starts to slow down. Both pages take a longer time to load (couple of seconds). Especially images are loading slowly. The problem occurs after a couple minutes after booting, it doesn't matter if I log in or not. I have the following broadcom nic: 02:00.0 Network controller [0280]: Broadcom (Hardware)