You can stream from two devices simultaneously at no extra charge. If you want 4 (altogether) you pay an extra $10.
So far I haven't noticed any degradation to any of the services while my wife is watching netflix in the other room, I'm on my laptop, she's facebooking on her phone, and I've got Viva going in the bedroom.
I guess it also depends on whatever speed internet you have. I've got 30 mb/s internet. Hope that answers your question.
You are allowed 2 devices or pay extra 10$ for 4. I have the box and use iPhone simultaneously with no problems. You can also link and unlike devices viva your dashboard on the web site. This means I can use as many devices as I want as long as I only watch no more than 2 simultaneously.
News 
Market Data 
Discover 
