TYPO3 Core

Reading on the internet there is no clear preference for either mbstring or iconv. I've found a post where somebody mentioned mbstring was a lot faster for him, while others mentioned that iconv is more likely to be shipped with PHP out-of-the-box than mbstring because it is more portable.

Thus I'd propose as autodetection-order: mbstring, iconv, (recode,) internal

For PHP6 "ICU" will be used. This is also available in current PHP-installations with the "intl"-extension already. But TYPO3 doesn't yet have ICU-support. (separate feature we might consider)

Problem is/was that there was no autodetection and if the setting was not set it defaulted to slow, handcrafted PHP-code. Christian Kuhn said he wanted to do something different in this area for 6.2 - but I'm not sure what finally arrived in master now.

Yeah, should be done. The configuration presets in install tool can detect mbstring and iconv (prefer mbstring), so this configuration can be selected with 1 click. During installation, the "best" alternative is chosen automatically (mbstring if available).