The InterpolationCompiler module contains optimizations that can tremendously speed up the interpolation process on the Simple backend.
It works by defining a pre-compiled method on stored translation Strings that already bring all the knowledge about contained interpolation variables etc. so that the actual recurring interpolation will be very fast.
To enable pre-compiled interpolations you can simply include the InterpolationCompiler module to the Simple backend:
I18n::Backend::Simple.send(:include, I18n::Backend::InterpolationCompiler)
Note that InterpolationCompiler does not yield meaningful results and consequently should not be used with Ruby 1.9 (YARV) but improves performance everywhere else (jRuby, Rubinius and 1.8.7).