void CalledThousandsOfTimesPerSecondFromManyThreads(float f)
{
static const boost::format Formatter("%0.3f");

GetStream() << Formatter % f << GetSomeOtherContent();
}

I'm pretty sure boost::format is not thread safe

If you think about what it's doing, every time you call operator% it's inserting a formatted item into it's internal state.

Think about what happens if you feed multiple elements via operator% and then call .str() - you get a std::string back - it has to store everything you've fed in internally so that you can access the formatted string later via .str()