std.math was split into a package with v2.097. This might imply that some little helpers (isNaN etc.) aren't inlined any longer without LTO (incl. Phobos), thus reducing performance of some math functions.
I am trying to call the fftw3 library using dpp however there is an issue with type translation in particular with function definitions. Within the C library mentioned, consider the function fftw_plan_dft_1d, within the library documentation the type signature is: