...and, according to this https://github.com/TechEmpower/FrameworkBenchmarks/blob/master/toolset/setup/linux/languages/dlang.sh#L11 , vibe is built with dmd, so there is some potential for improvement by just using GDC/LDC.
And it seems that current implementation doesn't use multicores (https://github.com/TechEmpower/FrameworkBenchmarks/pull/1723)