My iMac has turned into a PC - Soooooo slow! Help?

i dont know on earth happened imac yesterday, gone slow. feels i'm using windows based pc.
havnt added new programs/apps or anything.
lots of hang ups, programs load slowly, web browsers slow respond, constant spinning wheel of death.........even basic word documents take forever load up.
havnt done machine , theres on 100gb of free space out of 500gb on hard drive.
suggestions on cure?

 

look @ these , see, if problem:

performance tips mac os x

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have @ activity monitor (applications / utilities /) , select all processes , sort cpu see culprit may be.
check "system memory" tab see "page ins:", "page outs:" , "swap used:" are.

image below uses sorting cpu example

further reading:

mac os x: reading system memory usage in activity monitor
making of activity monitor
what activity monitor (or how take mac's pulse)​

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