There recently was an interesting SO question on F# Printf.* family of functions: http://stackoverflow.com/questions/11559440/how-to-manage-debug-printing-in-f It is known that these functions are very slow. Slow enough for most people to avoid them entirely, despite the advantages they offer in verifying argument types. What I did not know is that these functions are so slow that a few lines of simple user code can speed them up, without changing the interface: With this code I get from 2x to 10x [...]