Inexplicable LDC vs GDC speed difference

[Joseph Rushton Wakeling]

Following some code posted in d.learn, I've observed a bizarre and (to me) 
inexplicable difference in code speed depending on whether LDC or GDC is used as 
the compiler:
http://forum.dlang.org/post/mailman.1239.1360764028.22503.digitalmars-d-le...@puremagic.com
I'm using latest-from-GitHub versions of both compilers, compiled as release 
versions.

Anyone have any idea what could be the source of the speed difference?