>>44
performance mattering is subjective, and it depends on the application of the program. If rendering code for a video game takes a while and yields 1 frame per second game play, then that will ruin the experience for the player, making the game useless and inaccessible. If a script that is run twice a day to make back ups of some logs takes a couple minutes longer than it could have if it was written in C, then most likely, nothing bad will happen, and the choice of using the scripting language over C would pay off in terms of development time.
But back the functional and immutability stuff, some algorithms need to have mutability in order to keep their asymptotic bounds on their running times. There are efficient data structures that are completely immutable, but stuff like hash tables with arbitrary sequences of insertions and deletions need to have mutability.