Including crate increases build times by an order of magnitude about gltf

[NickDriscoll]

I had to learn that incremental compilation isn't enabled by default in release mode. Enabling it brought the build times back down to normal, but of course this is just masking the underlying problem.

[aloucks]

The primary culprits are: serde + serde_derive + gltf-json

serde is known to be somewhat slow to compile. The derived serialization and deserialization code is monomorhpized, resulting in quite a lot of generated code. On the other hand, miniserde uses trait objects (dynamic dispatch), so much less code is generated (and will result in some performance loss). It may be worthwhile to explore miniserde, but it has a substantially reduced set of customizations.

[Elabajaba]

gltf-json is the main slowdown from my testing. (with LLD on windows, 1.64 nightly)

Release build of the whole gltf repo (cargo +nightly build --release -Z unstable-options --timings=html)

Release build of bevy where gltf-json doesn't seem to block any crates from compiling, but still takes 40% of the overall time to compile.

[Elabajaba]

Checking this with cargo-llvm-lines, a release build of gltf-json expands to 577761 lines of llvm, with the vast majority of the time being spent in codegen.

edit: There's an open PR for serde_json that improves build times for me by over 50%.