Setting the frame rate to 30fps and disabling retina mode drops that figure to around 30 percent, which is still quite high. If you're using Duet Display on a laptop, you'll probably want to be plugged into a power source. The developer is aware of the CPU consumption issue and is investigating ways to reduce CPU usage. (Any such improvements will be distributed in a free update.)
Some may view Duet Display's $15 price as excessive, but if you need a fast extra display on your Mac at times, $15 is much cheaper than purchasing an additional display for occasional use (not to mention how hard it would be to travel with one).
I was impressed with the speed and ease of use of Duet Display. If the developer can solve the CPU usage issues, it will become a compelling solution for road warriors in need of speedy extra display space. In its current form, though, I'd recommend it only for extended use if you're connected to a power source.
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