I have a 3gb RAM laptop that has a 512 Nvidia quadro fx graphics card but blender is slow (lagging) when i subdivide my sculpture to make it smooth.Does anyone know why ?
The answer is actually pretty simple, subdividing creates more edges which causes it to need better hardware. Blender simply lags because your computer is not able to handle the smoothing.
3gb is not a lot, the operating system uses at least a third of that, if not more, just to keep the computer on, leaving you little to work with. When you subdivide you are creating more geometry, in exponential numbers. read: How do I know the number of polygons after adding a subdivision surface modifier without applying it?
Subdividing can easily push you to the end of your resources. When the system runs of of RAM it uses "virtual memory", meaning that it uses the hard drive to store things that do not fit in RAM, making the access of information much slower.
You need extra RAM. On top of that, you'd need something to cool down your laptop, especially when it can overheat. In fact, overheating can lead to crashes. If you are getting more RAM, also get a cooling tray like DefenderPad. https://youtu.be/L6FlK1VJetQ