In previs, there are often times when you have to animate objects flying through space: birds, helicopters, spaceships, angry flying monkeys, etc. With live action previs, however, the speeds at which your objects move have real-world implications when it comes time for the film crew to shoot, so you'll need to keep accurate speeds in mind. That way your previs team can inform the director and his crew how fast their helicopter will have to travel to catch up with all those angry flying monkeys.
Speed can be something a bit difficult to visualize for comparison, so here is the Speed Chart I ended up making:
If I had something like this before I started researching, it would have saved a lot of time. I hope this comes in handy for you at some point.
Apollo 10: http://www.history.nasa.gov/SP-4029/Apollo_10a_Summary.htm
Space Shuttle: http://www.space.com/12127-8-surprising-space-shuttle-facts.html
Bell X-1: http://airandspace.si.edu/collections/artifact.cfm?object=nasm_A19510007000
B-2 Bomber: http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=6
Cruise missile: http://www.wikiwand.com/en/Cruise_missile
P-51D Mustang: http://www.cafsocal.com/our-aircrafts/our-aircraft-and-history/north-american-p-51-mustang-%EF%BB%BFman-o-war/
News Helicopter: http://www.wikiwand.com/en/Airbus_Helicopters_H125_%C3%89cureuil