Pull size check out of the function. Profiling showed that a large number of samples (as in profiling samples, not sound samples) of Vector::FastSoundWrite() where in its prologue which was attributed to the early exit condition (basically just shuts the profiler up).

stepmania/src/RageSoundMixBuffer.cpp

@@ -62,6 +62,7 @@ void RageSoundMixBuffer::write( const int16_t *buf, unsigned size )
 #ifdef USE_VEC
 	if( g_bVector )
 	{
+		if( size )
 			Vector::FastSoundWrite( pBuf, buf, size, m_iVolumeFactor );
 		return;
 	}