concat.rb

require 'benchmark'

c = 100000
sa = 'abc'
sb = 'def'
sc = 'ghi'
Benchmark.bmbm do |x|
  x.report('<<') do
    c.times{ s = ''; s << sa << sb << sc }
  end
  x.report('+') do
    c.times{ s = ''; s = sa + sb + sc }
  end
  x.report('"#{}"') do
    c.times{ s = ''; s = "#{sa}#{sb}#{sc}" }
  end
end

gistfile1.txt

$ ruby -v concat.rb 
ruby 1.9.3p327 (2012-11-10 revision 37606) [x86_64-darwin12.2.0]
malloc_limit=1000000000 (8000000)
heap_min_slots=1000000 (10000)
Rehearsal -----------------------------------------
<<      0.090000   0.000000   0.090000 (  0.088408)
+       0.080000   0.000000   0.080000 (  0.084944)
"#{}"   0.090000   0.000000   0.090000 (  0.089944)
-------------------------------- total: 0.260000sec

            user     system      total        real
<<      0.090000   0.000000   0.090000 (  0.090973)
+       0.080000   0.000000   0.080000 (  0.080587)
"#{}"   0.090000   0.000000   0.090000 (  0.089658)

$ ruby -v concat.rb 
ruby 1.9.2p320 (2012-04-20 revision 35421) [x86_64-darwin12.0.0]
Rehearsal -----------------------------------------
<<      0.090000   0.000000   0.090000 (  0.088855)
+       0.100000   0.000000   0.100000 (  0.104386)
"#{}"   0.110000   0.000000   0.110000 (  0.102341)
-------------------------------- total: 0.300000sec

            user     system      total        real
<<      0.090000   0.000000   0.090000 (  0.088120)
+       0.100000   0.000000   0.100000 (  0.103511)
"#{}"   0.100000   0.000000   0.100000 (  0.102943)

$ ruby -v concat.rb 
ruby 1.8.7 (2012-02-08 MBARI 8/0x6770 on patchlevel 358) [i686-darwin12.0.0], MBARI 0x6770, Ruby Enterprise Edition 2012.02
Rehearsal -----------------------------------------
<<      0.090000   0.000000   0.090000 (  0.094898)
+       0.070000   0.000000   0.070000 (  0.073107)
"#{}"   0.080000   0.000000   0.080000 (  0.083200)
-------------------------------- total: 0.240000sec

            user     system      total        real
<<      0.090000   0.000000   0.090000 (  0.095308)
+       0.060000   0.000000   0.060000 (  0.065873)
"#{}"   0.080000   0.000000   0.080000 (  0.084841)