How can I set up RSpec for performance testing 'on the side'

I created rspec-benchmark Ruby gem for writing performance tests in RSpec. It has many expectations for testing speed, resources usage, and scalability.

For example, to test how fast your code is:

expect { ... }.to perform_under(60).ms

Or to compare with another implementation:

expect { ... }.to perform_faster_than { ... }.at_least(5).times

Or to test computational complexity:

expect { ... }.to perform_logarithmic.in_range(8, 100_000)

Or to see how many objects get allocated:

expect {
  _a = [Object.new]
  _b = {Object.new => 'foo'}
}.to perform_allocation({Array => 1, Object => 2}).objects

I managed to get what I was looking for via the following:

# Exclude :performance tagged specs by default
config.filter_run_excluding :performance => true

# When we're running a performance test load the test fixures:
config.before(:all, :performance => true) do
  # load performance fixtures
  require 'active_record/fixtures'
  ActiveRecord::Fixtures.reset_cache
  ActiveRecord::Fixtures.create_fixtures('spec/perf_fixtures', File.basename("products.yml", '.*'))
  ActiveRecord::Fixtures.create_fixtures('spec/perf_fixtures', File.basename("ingredients.yml", '.*'))
end

# define an rspec helper for takes_less_than
require 'benchmark'
RSpec::Matchers.define :take_less_than do |n|
  chain :seconds do; end
  match do |block|
    @elapsed = Benchmark.realtime do
      block.call
    end
    @elapsed <= n
  end
end

# example of a performance test
describe Api::ProductsController, "API Products controller", :performance do
  it "should fetch all the products reasonably quickly" do
    expect do
      get :index, :format => :json
    end.to take_less_than(60).seconds
  end
end

But I tend to agree with Marnen's point that this isn't really the best idea for performance testing.