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Well, more of the time it's because Ruby actually is, in fact, slow.

I though the implementations were slow or fast, not the languages.

Typically a language is considered slow or fast based on it's most popular implementation, sadly.

True, though language design can have implications on how fast things can be implemented.

Agreed, but it is still an implementation issue, because one can eventually discover ways to optimize such cases without changing the language.

Ruby is not, in fact, slow. This myth has been debunked so many times that I'll just leave the googling to you as an exercise.