> How would you go about writing a program/function that runs as close to native speed as possible on Fil-C?

Avoid pointer chasing. Use SIMD.

> How much more memory do GC programs tend to use?

I would estimate 2x

Fil-C has additional overheads not related to GC, so maybe it’s higher. I haven’t started measuring and optimizing memory use in anger.

> Curious, how do you deal with interior pointers, and not being able to store type info in object headers, like most GC languages do (considering placement new is a thing, you can't have malloc allocate a header then return the following memory, and pointer types can lie about what they contain)?

See https://fil-c.org/invisicaps