Tension in and of itself isn't a bad thing. Pain is though. If you've ever ran a mile or had a good workout you'd know what a "good" burn is. When I pick fast (sixteenths at 160+) for more than a minute straight, my arm tenses up quite a bit. The trick is to keep going, staying as relaxed as possible (which gradually becomes less and less) until your arm starts to burn (not like, fire burn, though). Then take a break. Do some left hand work or something. Keep doing that a few times a day and I'd say by the end of the week, if not much sooner, you'll be able to pick for longer periods of time and faster, before you start getting tired.