tls_codec performance

Hey @franziskuskiefer question for you, how is the current encoding / decoding performance? If there are places you think we can help make improvements let us know.

Originally posted by @tomleavy in https://github.com/RustCrypto/formats/issues/250#issuecomment-975838633

That's a good question. Before being able to answer the question we'd need to know what the performance is and understand if its any good.
There's a very basic benchmark here thttps://github.com/RustCrypto/formats/blob/4d0c0530be501e00dd1fbc25024db02eeffd669b/tls_codec/benches/tls_vec.rs. I wrote it to look at the performance impact of (de)serialising each element vs using the specialised ByteVec.

• Write benchmarks for real-world scenarios to measure performance.

There are two things that I think are worth looking at.

• Memory allocations are the most expensive operation. Are there any unnecessary allocations? Is any buffer resized unnecessarily?
• The existing benchmarks show that invoking (de)serialise for each element in a vector takes significant amount of time. Can this be optimised somehow?

Start with tls_codec/benches/tls_vec.rs and review the existing vector serialization benchmark. Add benchmarks for real-world scenarios, then investigate the allocation, buffer-resizing, and per-element serialization costs described in the issue. Done means the relevant performance characteristics are measured and any justified optimization is supported by benchmark results.