Add parallel frontend to the build performance guide

Author: Kobzol

src/doc/src/guide/build-performance.md

@@ -68,3 +68,21 @@ Trade-offs:
   - Speeds up build part of `cargo test`, but might increase its test execution part
 - ❌ Only available for [certain targets](https://github.com/rust-lang/rustc_codegen_cranelift?tab=readme-ov-file#platform-support)
 - ❌ Might not support all Rust features (e.g. unwinding)
+
+### Enable the experimental parallel frontend
+
+Recommendation: Add `-Zthreads=n` to the `RUSTFLAGS` environment variable[^rustflags], for example:
+
+```console
+$ RUSTFLAGS="-Zthreads=8" cargo build
+```
+
+This will enable the [parallel frontend][parallel-frontend] of the Rust compiler, and tell it to use `n` threads. The value of `n` should be chosen according to the number of cores available on your system, although there are diminishing returns. We recommend to set at most `8` threads.
+
+Trade-offs:
+- ✅ Faster build times
+- ❌ **Requires using nightly Rust and an unstable Cargo feature**
+
+[^rustflags]: You can configure `RUSTFLAGS` either as an environment variable when running Cargo, or through the [Cargo config](../reference/config.md#targettriplerustflags).
+
+[parallel-frontend]: https://blog.rust-lang.org/2023/11/09/parallel-rustc/