According to the 2010 Embedded Market Study the percentage of people who are not using Embedded GNU/Linux due to the lack of real-time capabilities went down from 25% in 2007 to 18% in 2010. I'll leave it up to you to interpret those numbers, but I strongly believe that it might be worth having a look at this subject.
Wat is real-time?
Real-time might, as the name implies, sometimes also be about timeliness (fast), but in my humble opinion it's usually about determinism. To be precise about event- and temporal determinism (event determinism meaning that you know the state of the outputs with a given set of inputs; temporal determinism meaning that you also know exactly the timing).
Coming back to real-time versus real fast - In case of an aircraft navigation system deadlines are in the tens of milliseconds, while for an airline reservation system the deadlines are at about 15 seconds.
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Getting real (time) about Embedded GNU/Linux