Would you mind submitting a bug report (with a reproduction case, if you're able) to the Clang team or mailing list? IIRC, they have an automated test suite for performance and regression testing; I'm sure they would greatly appreciate you taking the time to submit a test case where clang is producing egregiously slow binaries.
I could try, but although the difference seems localized to a fairly small number of functions, I haven't been able to precisely identify why it's so slow (there's no single inner loop etc that's slow), and shrinking it to something minimal would be hard...
