If you have books with undefined or unknown as language in metadata, iBooks 1.1.1 will ignore it and go with the English dictionary. In 1.1, it would tell you it couldn't open the dictionary for the unknown language (or words to that affect).
If you had a lot of books converted with Calibre before a certain version, they wouldn't have the language set to "en". It was fixed in later versions. The upshot is that with 1.1, the book would have to be re-converted by the new version of Calibre to get the proper language tag. With iBooks 1.1.1, the tag is bypassed (apparently), so the tag becomes moot.
For me, it's great news, as I still had dozens of books that would have needed re-conversion through Calibre.
Hard to say on the PDF speed - seems very responsive. It's now my default PDF viewer.