Metaprogramming definitely has a cost, but it's one that you can minimise if you have the ability to either invalidate and recompile code at run time, or if you

Metaprogramming definitely has a cost, but it's one that you can minimise if you have the ability to either invalidate and recompile code at run time, or if you can perform extensive whole program analysis when ahead of time compiling. The more extensive the meta programming you can do, the more work it is to implement this under the scenes. For example in Java you can change the visibility of fields, and you can load new classes, so that's not too hard to take account of, but in Ruby you can redefine methods, add refinements so they behave differently depending on where they are called, or radically change the inheritance hierarchy. Implementations like TruffleRuby can maintain high performance even with these features being used, but it's taken a lot of work to achieve that.