The comment on the call method of builder has a warning about performance. What's the alternative?

[timuckun]

# Call the Rack application generated by this builder instance. Note that
    # this rebuilds the Rack application and runs the warmup code (if any)
    # every time it is called, so it should not be used if performance is important.
    def call(env)
      to_app.call(env)
    end

[ioquatix]

You should invoke to_app and use the result of that instead.