Expose lagging on Mac mini

It is late 2012 model, 4GB RAM (now 10GB) i5 2.5 GHz Intel Graphics 4000. After 6 months of using it, the expose started to lag. (Not in the way when alot of programs is opened. ).It was around time I bought Magic Mouse.. does that have anything to do with the lagging. All softwares are up to date.
What do I do?