Take a peek at Paul's benchmark. Iterate and instantiated 100K empty objects in 10s. That's crazy. You're talking about something that would probably take 1ms in c#. You're talking about a performance deficit four orders of magnitude large.

Regardless, I think you've entirely missed the point. The point is not that the O/RMs generate inefficient queries. They may not be perfection, but for what you're asking of it, by and large the queries are not the issue. We're talking about raw method-dispatch.

This simple benchmark takes about 600ms on my MacBook Air:

  require 'benchmark'
  puts Benchmark::measure {
    i = 0
    1_000_000.times { |x| i += x }
  }

  using System;
  using System.Diagnostics;

  namespace dispatch {
    class MainClass	{
      public static void Main (string[] args)	{
        var s = new Stopwatch();
        s.Start();

        var x = 0;
			
        for (int i = 0; i < 1000000; i++) {
          x += i;	
        }
			
        s.Stop();
			
        Console.WriteLine(s.Elapsed.ToString());
      }
    }
  }

You could argue that it's not exactly identical code, but it's fairly idiomatic I'd think for each language.

The point is, the sort of performance deficit you carry with Ruby has real consequences. You don't have to dig very deep at all until such concerns are no longer academic. There's financial applications I've worked on in c# doing transaction reporting with 1,000 rows or so per page that would simply put be at a severe handicap under Ruby and it has nothing to do with the RDBMS.

I'm certainly not gonna give Ruby up any time soon. But it's important as a developer to at least be aware of the shape of the box you live in I think.