Performance of findCirclesGrid on larger grids

edit

I have found that findCirclesGrid takes a long time (on the order of minutes) on reasonably large grids such as 23x18, contained in 4 MP images. I'm using default parameters, and the images are simple, i.e. contain only circles and nothing else. The function findChessBoardCorners suffers no such degradation. Versions tested are 2.3 and 3.2.

Can the execution time be improved with different parameters?

Furthermore, I have noticed that the flag CALIB_CB_ASYMMETRIC_GRID does not work on 23x18, and CALIB_CB_SYMMETRIC_GRID must be passed to ensure detection. Is this expected?

EDIT: Sample image at https://imgur.com/a/STKAs