Typically the generation of the uniform random numbers can be made quite cheaply, especially if the resolution is not critical (e.g. if uniform floats are enoug

Typically the generation of the uniform random numbers can be made quite cheaply, especially if the resolution is not critical (e.g. if uniform floats are enough and not uniform doubles etc) or if the quality of the randomness is not critical, and in these cases discarding half of these will win versus other costly operations.