How would you improve Performance of Mac OS X?

There are many things users can do to increase the speed of their Macintosh. There is, of course, no way to make an older system perform as well as a modern system, but it is possible to optimize the performance of an existing system.

My one reader asked a question that " Since approx. one year I'm using a SSD in my Mac mini. After having it built-in I remember a speed-improvement compared to the default hard disk, but today it (the Mac mini) feels sluggish. What can I do to improve the performance?

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