Tracking issue for improving build speed

[smklein]

• Consider breaking up big Nexus 🔨 #1154
• [performance] Decouple dependencies between Nexus / Sled Agent #1156
• slow incremental rebuilds of Nexus tests #1122

[smklein]

Some notes for others:

cargo build --timings
cargo check --timings

Both show different (but equally useful!) results

[smklein]

Also found some good tips for profiling:

https://blog.rust-lang.org/inside-rust/2020/02/25/intro-rustc-self-profile.html
https://fasterthanli.me/articles/why-is-my-rust-build-so-slow