Dang, that's a hefty build. I suppose the optimizer in the compiler probably goes absolutely nutso on looking for optimizations in that code.
That Unreal "compilation" isn't your ordinary C compiler most of the time. First of all, most "coding" in Unreal isn't code in the traditional sense, but visual assembly via Blueprints. And lots of game data is just that, data. Yet it still needs to be converted from some abstract format that is as machine independent as possible into something that runs devilishly fast on the target platform underneath. Good chance that includes generating abstract shader language that then again needs to be compiled into a binary format for the hardware.
I haven't really gone deep, I was mostly interested to see if I could easily plug in external code e.g. to make Dinos in Ark send what they saw with their eyes to some machine learning model and then translate its recognitions into behavior and actions.
Turned out that couldn't be done in a week-end or three, so I stopped there, but it was still amazing to see and dabble.
I've found CPU emulators drive the compiler the most nuts, but 3D code and things involving matrix math give the compiler a nice workout too. I imagine it checks if it can use MMX/SSE/AVX/etc. instructions to speed things up; potential speedups from loop unrolling; seeing if reordering instructions could yield some massive speedup; sees if some array is being looped through top-to-bottom when left-to-right would be much faster (or vice-versa), and so on. I mean the compiler 'always' checks for those things but a lot of code it can probably just see they don't apply and skip them, the kind of work 3D code does it probably carefully runs through every single check.
I completely agree, modern CPUs have single vector instructions, that would require pages and pages of normal C code to emulate functionally or some other RTL or similar format to describe functionally: even if there is a rough functional equivalent, you can't just translate from one to another.
That's why Unreal attempts to start with a very high abstraction, which has a perhaps better defined visual goal than those HPC applications you were trying to emulate.
And then it does the magic that makes it so valuable.
It's all open and free to check out, so if you find the time, have a go! Better than lots of doom-scrolling...