[adrianN]

You can try an experiment and build an application like Firefox from source with disabled optimizations. I bet you'll notice a massive difference.

Even more important are things that run in datacenters on thousands and thousands of machines. Even if you suppose that optimizations make only a minuscule difference on the scale of today's infrastructure 5% fewer machines can save huge amounts of electricity.

[ArkyBeagle]

Why would I want to build Firefox from source? I don't build things that run on thousands of machines. Last thing I ran any code on had a 2500 HP diesel; the electronics were in the noise.