Add notes about target-cpu to README

arrow/README.md

@@ -82,3 +82,17 @@ cargo run --example read_csv
 ```
 
 [arrow]: https://arrow.apache.org/
+
+
+## Performance
+
+Most of the compute kernels benefit a lot from being optimized for a specific CPU target.
+This is especially so on x86-64 since without specifying a target the compiler can only assume support for SSE2 vector instructions.
+One of the following values as `-Ctarget-cpu=value` in `RUSTFLAGS` can therefore improve performance significantly:
+
+ - `native`: Target the exact features of the cpu that the build is running on.
+   This should give the best performance when building and running locally, but should be used carefully for example when building in a CI pipeline or when shipping pre-compiled software. 
+ - `x86-64-v3`: Includes AVX2 support and is close to the intel `haswell` architecture released in 2013 and should be supported by any recent Intel or Amd cpu.
+ - `x86-64-v4`: Includes AVX512 support available on intel `skylake` server and `icelake`/`tigerlake`/`rocketlake` laptop and desktop processors.
+
+These flags should be used in addition to the `simd` feature, since they will also affect the code generated by the simd library.