CFITSIO has been carefully designed to obtain the highest possible speed when reading and writing FITS files. In order to achieve the best performance, however, application programmers must be careful to call the CFITSIO routines appropriately and in an efficient sequence; inappropriate usage of CFITSIO routines can greatly slow down the execution speed of a program.
The maximum possible I/O speed of CFITSIO depends of course on the type of computer system that it is running on. To get a general idea of what data I/O speeds are possible on a particular machine, build the speed.c program that is distributed with CFITSIO (type 'make speed' in the CFITSIO directory). This diagnostic program measures the speed of writing and reading back a test FITS image, a binary table, and an ASCII table.
The following 2 sections provide some background on how CFITSIO internally manages the data I/O and describes some strategies that may be used to optimize the processing speed of software that uses CFITSIO.