mb_detect_encoding is very slow

[schlessera]

Profiling the current test suite showed that the use of mb_detect_encoding() made up more than 40% of the execution time of the entire test suite.

We should:

• [ ] ensure we only use it if really necessary to decrease the amount of times it runs
• [x] document that it is slow and that documents should therefore use a proper charset meta tag to begin with
• [ ] make sure a charset is set in integration like the WP plugin to avoid its use altogether in those cases