results

Rehearsal -------------------------------------------------
string interp   0.200000   0.000000   0.200000 (  0.202780)
array join      0.520000   0.020000   0.540000 (  0.540294)
---------------------------------------- total: 0.740000sec

                    user     system      total        real
string interp   0.210000   0.000000   0.210000 (  0.216187)
array join      0.520000   0.000000   0.520000 (  0.518884)

string-bench.rb

require 'benchmark'

Benchmark.bmbm do |x|
  n = 1_000_000
  subdomain = 'snowdevil'

  x.report('string interp') do
    n.times do
      "#{subdomain}.myshopify.com"
    end
  end

  x.report('array join') do
    n.times do
      [subdomain, '.myshopify.com'].join
    end
  end
end