clang 3.0 compiled with mingw gcc 4.6.2 is slow.

I got the llvm/clang 3.0 source code from the llvm download page. And
compiled it with mingw gcc 4.6.2. Here follows my building steps:
         0. AddPath
         1. cd to source root
         2. mkdir BUILD
         3. cd BUILD
         4. cmake -G "MinGW Makefiles" -DCMAKE_BUILD_TYPE=Release ..
         5. make
It takes more than 5 seconds to invoke clang/clang++ commands, even if
the command is just "clang++ --version".

Is there something wrong in my building steps? Or mingw version of
clang++ do have performance issue?

  Thanks in advance for your help.