Russell Owen added a comment - 14/Apr/16 2:35 PM - edited Comparing GWCS and PyAST in Python The model is a 2nd order 2D polynomial for actual pixels to mean pixels, followed by a pure TAN WCS for mean pixels to sky. Times are in µsec and were measured on a mid 2012 Macbook Pro with a 2.6Ghz Intel Core i7. # points GWCS PyAST GWCS PyAST ratio time time time/pt time/pt 1e2 15200 62.3 152 0.623 24 1e4 18900 2610 1.89 0.261 7.2 1e6 346000 269000 0.346 0.269 1.3 The code can also measure the use of PyAST's tranGrid , which interpolates, but the results are difficult to interpret because the timing, naturally, is dependent on the tolerance. In any case, the timings above are more relevant to warping, since warping does its own interpolation. Warping is especially interesting because it is requires fast WCS for good performance (though interpolation ameliorates this) and because it is not easily vectorized (suggesting it would be difficult to get good performance in python).