This should also speed up DateTime. This will not help 1.9 performance.

The attached file is based on MRI 1.8.6 rational.rb.

 > ruby rational_performance.rb 
                                              user     system      total        real
test_it                                  32.930000   3.030000  35.960000 ( 35.971832)
test_it                                  33.840000   2.910000  36.750000 ( 36.758585)
test_it ks_rational                      29.110000   2.460000  31.570000 ( 31.572762)

Overview:

• case x; when Foo; ...; end is faster than if Foo.kind_of?(x).
• Avoid recuring on ephemeral objects.
• Avoid local variables, in-line expressions.
• Avoid return in tail position.
• String interpolation: "#{x}/#{y}" is faster than x.to_s + "/" + y.to_s.
• Implement #-, #zero?, #nonzero? natively.
• #abs returns self if > 0.

MRI 1.8.7 patch to follow shortly.

• ks_rational.rb
• rational_performance.rb