As lalimacefolle said, the key point in order to increase your speed together with your accuracy is to use the metronome. And this particulary true for arpeggios.

Trys to do it slowly and increase step by step the metronome, days after days ... a lot of works and you will surely do it ...

You can check some of the exercices (tabs illustrated with an mp3) I put on my website : http://www.guitarweb.net