helpers.rb

RSpec::Matchers.define :finish_within do |timeout|
  match do |block|
    begin
      Timeout.timeout(timeout) do
        Benchmark.realtime{ block.call } <= timeout
      end
    rescue TimeoutError
      false
    end
  end
end

spec.rb

it "should respond in 0.1s" do
  expect {
    visit '/'
  }.to finish_within 0.1
end