The trick to optimizing code is that you should never attempt to do this without having a full understanding of what the change you're proposing will affect. Just having a feeling or a vague idea of how it might be a good idea isn't good enough.
While SoC-based platforms with a full OS tend to give you a bit more leeway, for MCU platforms, it's essential that you understand what the addition of a single keyword or use of a different data structure to store some information will mean.
The worst thing to do here is to assume that optimizations that you've used on SBCs and desktop systems will have a similar effect on an MCU platform. Due to the modified Harvard architecture and various quirks of platforms such as AVR, these are most likely to backfire or, if you're lucky, just be ineffective.
Here, the application notes provided for the (MCU) platform are useful to understand how the hardware can be optimized. The take-away message here is to do your research before making optimizations, just as one doesn't just start writing code without considering the project design.