I just tried the new trunk version of clang. The bug I previously reported about typing a column in a .m file seems solved, or at least it's not been reproducible yet. Thank you very much for helping with it.
Unfortunately it appears that clang_reparseTranslationUnit() has become much slower in the following condition (reproducible by recompiling the source I sent together with the newest version of lib clang):
Open a .m file.
Try typing inside the scope of a class implementation. This seems fast enough.
Try typing outside the scope of a class implementation, for example in a line below a #import directive. clang_reparseTranslationUnit takes around half a second to complete in this case, exactly as I reported with .h files .