One big subscriber vs. multiple small subscribers?

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If you're worried about the overhead of multiple subscribers for high bandwidth/frequency topics like images and pointclouds, what you want is probably nodelets. From the wiki page:

Nodelets are designed to provide a way to run multiple algorithms on a single machine, in a single process, without incurring copy costs when passing messages intraprocess.

However, it has been said that "premature optimization is the root of all evil." I would recommend that you first try having your nodes subscribe independently as normal and see if you actually need to do anything fancier.