system ~/projects/jruby $ jruby -rbenchmark -e "5.times { puts Benchmark.measure { 1000000.times { s = 'foo'; s.instance_exec { to_s } } } }"
  5.330000   0.340000   5.670000 (  2.279000)
  3.630000   0.330000   3.960000 (  1.693000)
  3.490000   0.350000   3.840000 (  1.671000)
  3.510000   0.330000   3.840000 (  1.654000)
  3.470000   0.330000   3.800000 (  1.644000)

system ~/projects/jruby $ jruby -rbenchmark -e "5.times { puts Benchmark.measure { 1000000.times { s = 'foo'; s.instance_exec_light { to_s } } } }"
  1.290000   0.050000   1.340000 (  0.675000)
  0.310000   0.020000   0.330000 (  0.288000)
  0.280000   0.020000   0.300000 (  0.281000)
  0.290000   0.030000   0.320000 (  0.284000)
  0.290000   0.020000   0.310000 (  0.281000)