is it possible, that the Mail import do not work on the new OS
First, just to say it – if you are using your “real” mail data or accounts with a beta OS, not a great idea IMO. You didn’t say you were, but just in case …
On the mail plug-in issue – it’s very possible that the changes in Yosemite do not work with some features of existing applications that have not been adapted yet to Yosemite. On the other hand, almost every time Mail.app gets updated, you need to reinstall the plug in. Which is why this topic gets covered over and over here. A quick search of the forum reveals:
don’t use the “real”. But very interesting is that DTPO is working much faster. I don`t know why, but loading DB is really fast now.
In Yosemite, or in general. The latest build should be faster in general. If it’s even faster than Mavericks, that’s a nice perk!