### ruby 1.8.7 (REE)

    irb(main):031:0> start = Time.now; 10_000_000.times { 'foo' }; puts Time.now - start
    => 2.154165
    irb(main):032:0> start = Time.now; 10_000_000.times { "foo" }; puts Time.now - start
    => 2.177029

Remind me again, why do I care about 23 milliseconds for every 10 million string
loads?  Methinks that just one instance of needing to change single quotes to
double quotes so you can do some interpolation that you hadn't originally
foreseen will take more time than a month of ruby loading double quotes, unless
its on some superduper hyper sensitive performance critical inner loop (in which
case, are you sure ruby is the right language for you?).  Am I wrong?

Oh, and how about other versions of ruby?

### ruby 1.9.3-rc1:
    irb(main):002:0> start = Time.now; 10_000_000.times { "foo" }; Time.now - start
    => 1.551125075
    irb(main):003:0> start = Time.now; 10_000_000.times { 'foo' }; Time.now - start
    => 1.535057393

Again, why do I care about 16 milliseconds for every 10 million string loads?

### Rubinius 2.0.0-dev:
    irb(main):004:0> start = Time.now; 10_000_000.times { 'foo' }; Time.now - start
    => 0.745409
    irb(main):005:0> start = Time.now; 10_000_000.times { "foo" }; Time.now - start
    => 0.807167

This is the biggest difference yet.  An awe inspiring 55 milliseconds per 10
million string loads.  Again, a single instance of changing to double quotes
because of an unforeseen interpolation will erase that entire performance
penalty.

### JRuby 1.6.4:
    irb(main):003:0> start = Time.now; 10_000_000.times { "foo" }; Time.now - start
    => 0.743
    irb(main):004:0> start = Time.now; 10_000_000.times { 'foo' }; Time.now - start
    => 0.757

Not only is JRuby just plain faster overall, it's *also* faster for double
quotes than single quotes!

### In conclusion

These benchmarks are complete BS.  If you have profiled actual production code
and see a significant speedup in a performance critical section, then go ahead
and make the change.

But I like my default design choices to be *useful*.  I like to optimize for the
programmers first and the computer second.

Anyway, if you *really* care about pre-optimizing microseconds: use double quotes by
default and switch to JRuby.