Original `poly` functions are very good and useful, but they do a lot of not-always-necessary work (matrix condition checking), especially when one is in the middle of a tight `for`-loop. `poly_fast` functions do not do these checks, but assume the user is sane and knows what they're doing. It is recommended to **first** write one's code with normal `poly` functions, and then switch to the `poly_fast` once the code works fine. This can be done by simply replacing the calls to `polyfit` and `polyval` with ones to `polyfit_fast` and `polyval_fast`.