It is important to note that message complexity has a lot to do with proto performance. Anyone with a basic understanding of algorithms could tell you that, but I prefer to be shown things so here's a repo with some examples of Python proto serialization performance so you can see how it does as message complexity increases:

https://github.com/theRealWardo/proto-bench

So yes, if your application has to send a lot of structured data around, you will end up paying a non-trivial serialization cost. On the other hand, if your application is sending really simple messages then I highly doubt proto serialization is the performance bottleneck you should focus on.