Default Hide front buttons in app - fullscreen performance drops

LG D958, Stock 4.2.2. When using useful function "hide front buttons" in specific apps, I've noticed some problems with performance, especially in graphics-rich apps. Tried gfxbench, onscreen mode 25.6 FPS vs 33.1FPS. The same with Real Racing 3 etc and it become difficult to play. Seems like stuttering in render. I doubt that the extra 72 pixels may affect performance so much. So it would be a disappointment if the problem is not corrected in 4.4 update, this function become useless in 3D apps . Is it possible and can anyone send report to LG about this bug? Thanks.
LG D958 TW, Stock Current ROM