Raspberry Pi is a single board, ARM based mini-computer which costs as low as $35. This board also features Broadcom VideoCore graphics that is enough to play 1080p HD videos. Though the processor is not very powerful, you can play some medium and low end games successfully on it. The device lacks open-source drivers and people have to rely on proprietary drivers for advanced graphics. As proprietary drivers integration is poor, most apps that require high graphics usage, lag in it.
Things may change, as developers are busy making an open-source source driver for it. As according to a news from Phoronix, there is some major announcement coming soon. Its not clear whether the announcement will come from independent developers or from devs at Broadcom, but its clear that this will bring some sort of relief to graphic enthusiasts who want to buy Raspberry Pi.
Open-source drivers will bring a lot of improvements and chances are, more games and graphic related tasks will be able to run lag free in Raspberry Pi. This will only be possible if Broadcom developers agree to give free access to the source code of their drivers.