doc: Add section on how to achieve best performance

[aaronbojarski]

The idea behind the new default was to optimize for a good first build/run experience also on platforms that don't support AES-NI.

I think that is a reasonable argument. Let's go with an easy first build and use OpenSSL by default. But we should have a section in the documentation that describes how to achieve the best performance. (We can open an issue for that and handle it later)

Originally posted by @fstreun in #19 (comment)