require "benchmark/ips"

object = Class.new {
  module_eval "def evaled_method; end"
  define_method(:defined_method) {}
}.new

Benchmark.ips do |x|
  x.report("module_eval")   { object.evaled_method }
  x.report("define_method") { object.defined_method }
  x.compare!
end

Calculating -------------------------------------
         module_eval    98.664k i/100ms
       define_method    93.809k i/100ms
-------------------------------------------------
         module_eval      6.794M (± 8.1%) i/s -     33.743M
       define_method      4.588M (± 6.7%) i/s -     22.889M

Comparison:
         module_eval:  6793763.3 i/s
       define_method:  4587570.1 i/s - 1.48x slower