[kazinator]

Fexprs have performance problems over macros because fexprs are interpreters whereas macros are compilers. A macro compiles the semantics that the equivalen fexpr would interpret. In 2019, compilation is still generally faster than interpretation.

Fexprs are theoretically interesting in one regard: they can avoid hygiene problems. A fexpr explicitly evaluates material in some explicit environment, so it can work out which environment is used for what. It has straightforward access to its own local material that doesn't interfere with the interpretation of the incoming form, that is done using its own environment. A loop that has an implicit counter which is invisible to the program, if implemented as a fexpr, can just use the fexpr's own local variable as a loop counter, and thus doesn't have to insert a hidden gensym into a code template. A fexpr is not confused if the interpreted form defines a local function called list, even if the fexpr uses list. Macros combine everything into one clump of code, so achieving the same environmental separation requires significant hoops (which are omitted entirely in traditional Lisp macro systems).