FYI, it appears that the patch I just submitted PERF: NeighborhoodOperatorImageFunction avoids copy ConstNeighborhoodIterator http://review.source.kitware.com/#/c/22959/ significantly improves the performance of the HoughTransform2DCircles filter. I reran the test that I posted at Dec 10, 2017 at Hough Transform 2D Circles Image Filter GetCircles patch. and saw a performance improvement of more than 30% (duration going from 45 seconds before the patch, to less than 30 seconds).