BLAS uses low level routines that are difficult to replicate in C. Some of the stuff is written in FORTRAN so as to avoid aliasing issues inherent to C arrays.

BLAS uses low level routines that are difficult to replicate in C. Some of the stuff is written in FORTRAN so as to avoid aliasing issues inherent to C arrays. Some implementations use direct assembly operations. It is heavily optimized by people who really know what they're doing when it comes to floating point operations.