Why on board graphics is unfit for gaming/rendering?
The difference between dedicated and integrated device is in resources and power. Dedicated have own resources integrated share them with various tasks. Integrated is less parallel but has (commonly) higher frequency. Integrated card uses CPU and RAM while dedicated has own RAM and own cores (far more than dedicated one).

This makes integrated cards unfit for real graphics, besides rendering part there are also tasks like controlling the game, running AI, keeping buffers of done tasks etc.