It's faster because you combined the glue-code which was necessary for the used compiler: Green Hill.
The used Green Hill C-Compiler wasn't able at that time to fetch arguments from registers directly; instead, assembler written glue-code had to be provided to move the register arguments into the corresponding C-compiler expected stack arguments, because at that time, two ABIs were used; the Amiga ABI (register arguments) and the m68k ABI (stack frame). While the Green Hill C-Compiler did only support the m68k ABI (as well as later the earlier versions of the Aztec and Lattice compiler) the workaround was at that time to use assembler generated glue-code - till later so called PRAGMAs and AMICALLs were introduced in order to support the Amiga ABI without using glue-code.
If one gets the chance compiling Intuition newly with current compilers (SAS/C, gcc or vbcc) the by you combined glue-codes gets completely removed, just because these compilers support the Amiga ABI. Of course there would be a little speed-up, but honestly, it would be so marginal that no one would notice it.
Rewriting the Layers-library from ground up like THOR did, inclusive a new algorithm, is far more noticeable.
Next, touching and optimising a beast like Intuition requires really deep insights into how this big state-machine works - and frankly, there are only a handful people who I would trust in that they would have the knowledge and skills for this kind of task.
Fortunately for me as user of the AmigaOS 3: No one is allowed to break the functionality of Intuition, because that what I would expect if once it would become unmaintained Open-Source...
