Running and downloding slow,freez totally.

There are many builds of the iMac, you haven't shared enough information about your specific machine for anyone to be able to offer useful help. While waiting for you to completely ID your Mac, I will tell you this:

The amount of RAM and HD free space significantly impacts the operation of a computer, more than the OS. Insufficinet RAM slows comptuers, Failing HDs slow computers. Full HDs slow computers. Upgrading the OS, like putting premium gas in a regular 4 cylinder auto, may not improve performance. In fact if your machine fails to easily meet the OS System Requirements it could slow machine, especially if it's done over a corrupted/failing HD.

Running Utilities->Disk Utility:Disk First Aid->repair disk and repair permissions, is a good place to start diagnosing, eliminating HD failure. So is running your AHT (Apple Hardware Test) disk. Sharing what you find, along with the specifics of your machine, will help us help you.

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