This is one of the reasons. Ruby 1.9 is usually compiled with 'gcc -O3' option, but Ruby 1.8 is compiled with 'gcc -O2'. '-Ox' option is the optimization option for the compiler. They do not recommend to compile Ruby 1.8 with '-O3'.
http://www.loveruby.net/ja//rhg/cd/build.html (Japanese, Aoki-san)