ret == 0 means EOF. Don't update the texture; it'll loop and update it again right away. This caused an update before the previous parallel unlock had finished, which apparently triggered the slow path in the driver and caused a 15ms skip.

stepmania/src/arch/MovieTexture/MovieTexture_Generic.cpp

@@ -460,7 +460,7 @@ int MovieTexture_Generic::GetFrame( float fTargetTime )
 
 	int ret = m_pDecoder->GetFrame( m_pSurface, fTargetTime );
 	if( m_pTextureLock != NULL )
-		m_pTextureLock->Unlock( m_pSurface, ret != -1 );
+		m_pTextureLock->Unlock( m_pSurface, ret > 0 );
 	return ret;
 }