Ruby is slower than C++ at a number of easily measurable tasks (e.g., doing code that is heavily dependent on floating point). This is not very surprising, but enough justification for some people to say that “Ruby is Slow” without qualification. They don't count the fact that it is far easier and safer to write Ruby code than C++.
The best fix is to use targeted modules written in another language (e.g., C, C++, Fortran) in your Ruby code. Those can do the heavy lifting and your scripts can focus on higher level coordination issues.
