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You should know that you are trying to use roslaunch in a way that is unusual. Typically ros nodes do not start and stop very much. Rather, the idea is that roslaunch starts all the important nodes at the beginning, and then the nodes wait quietly for work to do. I'm not saying your approach will not work, but you should expect slow performance if you dynamically execute roslaunch. A typical ros node does not start up very quickly - it might take a second or two; conversely a node that is waiting for work can react in 10 or 20 milliseconds.

I would implement the kind of change you want in an always-running ros node that monitors topics for activity it cares about.