I'm assuming by the words gas and slow you are referring to oxy/acetylene welding?
If you achieve the correct flame and use the correct nozzle for the work piece you'll be surprised at just how quickly the puddle moves along .
You have to get the aforementioned right so that you limit distortion to the minimum possible! There are simple AIDS you can use that help reduce distortion to a negligible amount, especially if you weld 1mm sheet, which is approx thickness of say a Morris minor door skin for example . BUT you'll be surprised how LOST and caught up with the quite MESMERIZING characteristics of oxy/acetylene welding half an hour an hour seems to be like just minutes
Chris