The League of Legends stream at the main page has no lag.
Okay, but that stream is low quality and mine is 720p 60fps.
Just beacuse one stream doesn't lag doesn't mean all of them shouldn't. Every stream is different.
I stream at 3000kbps, which is 3Mb/s, to put it in the terms that your speedtest.net result does. Since you're from the Netherlands, your 7.37mb/s is probably more like 1.5-2mb/s to an American server, which is where the my stream comes from. It seems most likely to me that your connection just isn't fit for a high quality stream.
Try doing a speedtest to a sever on the east coast in the United States. If you're still getting better than 5.0Mb/s, then I'll need to figure out what's wrong. --- http://www.twitch.tv/vraage || GT: v Raage 3770K@4.7GHz || GTX680 FTW 4GB || 8GB 2133MHz || Maximus V Extreme || 250GB SSD || 2TB HDD || H100i (CPU) + H55 (GPU)