Since the device is a low price handset, you will have to compromise on certain features. The Galaxy S Duos comes with the single core 1GHz Cortex-A5-based processor on a Qualcomm MSM7227A Snapdragon S1 chip. The RAM is of 768MB that works fine however, it is not enough for gaming and other fancy stuffs.
Although, the device is not perfect but it works well enough. Although, there is a delay browsing around menus, and ICS lacks the visual buttery polish of Jelly Bean. Moreover, when you open an app such as the Contact, there is an almost ubiquitous excruciating lag but on the whole the Galaxy S Duos feels snappy.
With the 4GB of internal storage that is already used by the operating system, you will only have 2GB left for apps and other stuff therefore, you can expand it with the help of a microSD card. You can add up to 32 GB by inserting a microSD card.