The algorithms deployed in these kind of codecs take into account not only human vision and mathematical laws of information, but also nitty-gritty details of how computers work, which are optimally exploited by directly having humans write detailed assembly rather than a compiler make a best guess and effort.

Because it's 5 times more complex, you need to get the maximum performance available. Therefore more ASM than ever.

Rust does not bring more performance. Just more safety.

Encoder and decoder writers frequently need extremely fine grain control over SIMD instructions in order to get good performance.

The way they weave these instructions can be very hard to express with a high level language.

Further, there's a ton of work with arrays and importantly parts of arrays. They can, for example, need to extract every other element up to 1/2 the array. Unfortunately, rust has runtime array bounds checks which make writing that sort of code slower. The compiler can elade those checks, but usually only in simple cases.

The authors would be writing a bunch of unsafe rust to get the performance they want and rust makes that more painful on purpose.

I like rust, but C/ASM really is the right choice here. This is one of the few cases where rust's safety is a major detriment.

The ffmpeg devs have said many times in public that they routinely get speedups of 10x or more over C code. I'm not a reputable source on this myself but I highly recommend looking into their channels, mails, or posts.

yes it makes sense to use C/ASM here, but if you're curious, there is a rust port of dav1d named rav1d: https://github.com/memorysafety/rav1d

it's not much slower than the original C/ASM implementation (last i checked ~5%?) but that matters here

Go ask FFmpeg what they're writing their encoders and decoders in.

Yes? There is 5x more code to optimize the ASM for.