There was an interesting conversation held on the ITK Community mailing list regarding the time it takes to initialized the image registration framework:
The conclusion was that a significant amount of time is consumed by the slow DiscreteGuassianImageFilter, the SmoothingRecursiveGaussianImageFilter can provide improved performance, but slightly ( insignificant? ) different results.
I would also like to note, that when a gaussian filter, then a “shrink” filter is done, more efficiency can be gained by doing the gaussian in 1D, then shrink in 1D, repeat. Consider a case where you are shrinking by 10x10x10, This amount of work done by the gaussian can be reduced by over 100x.