With USE=-cpudetection I get ... Compiled with runtime CPU detection - WARNING - this is not optimal! To get best performance, recompile MPlayer with --disable-runtime-cpudetection. ... I think the default is on, so it should explicitly disable cpu detection unless cpudetection use flag is used.
Nevermind. I had USE=bindist which is what was actually causing it. Sorry guys.
Closing... :)