I think you can avoid taking the square root in the distance formula (by squaring the comparison term), so this actually could be the fastest way possible to reach the effect, because of all the sampling, the points outside are a minority. What the tweets mention instead is you need to take the square root, that is the slowest thing in the expression.
That is indeed the fastest way in my experience (at least on the CPU). It's very unlikely to have more than a few rejections, so any method that requires evaluating trigonometric functions cannot compete.
