I read this more in the context of ‘how to do it quickly’. There was discussion a long time ago about various tricks from contributors, some of it made it to the wiki pages. Some of it likely doesn’t apply by now anyway so I wouldn’t heed it much.
The math parser (e.g. in the fill command) used to be avoided but it’s quite good now, which also removes two other problems; using loops over an image (like repeat… done) isn’t required and resorting to ‘abusing’ the 3D commands is no longer needed. If you’re writing filters from scratch and you really need speed I find sticking to ‘native’ commands (i.e. only running C++ code not G’MIC script) is best. Even then you can get a lot done by mixing that with the math parser. With larger files using shared buffers can make quite a large difference due to not having to copy data around.