How to reproduce:

1. Open a C++ file
2. Press Ctrl+S (Save action)

What happens:
QtCreator's GUI interface freezes for about half a minute, maybe slightly less.

I have run QtCreator with clang debugging output (export QT_LOGGING_RULES=qtc.clang*=true export LIBCLANG_TIMING=1) - you can find its output here.
Strings like qtc.clangbackend.timers: ParseSupportiveTranslationUnitJob needed 10747 ms look suspicious.

Here you can find perf top output. And here are some interesting function from the perf top log:

1. libClangFormat.so
   1. clang::SourceManager::getDecomposedLoc
   2. clang::SourceManager::isBeforeInTranslationUnit
   3. clang::format::FormatTokenLexer::getNextToken

PS. Note that I DO use extensively clang-format as a part of the Beautifier plugin (I assign a shortcut to the action and invoke it manually before saving) and I do NOT have any problems (like hangs) and the code is formatted very quickly.