My new iMac (21.5" late 2012) runs slow and unresponsive

Hi,

Just bought a new iMac 21.5" (late 2012) about a week ago. I found that this iMac runs slow and really unresponsive. It took 10 second (apprx) to open system preferences, and more other. I found this very annoyed, compare to my last iMac. I know this new iMac has only 8GB memory (only!) and cannot be replaced. My stupid question is, if cannot modify my memory, than what should I do to make my iMac as responsive as I expected like the previous one?

If one of you guys can help me with this issue, I would be much more appreciate your kind.

Thx.