Q: slow imac 21.5 late 2012

Have you done as the original Poster did and access the router for your iMac and  change the channel of the WiFi router?

Already did that a few years ago. And even if I wouldnt have done that, why would there be such a huge difference between my old and new mac, using exactly the same connection?

In your network settings try creating a new location and see if that helps with your issues.

Also, have you tried changing to all the different channels on your router?

Just curious.

Make a New Location, Using network locations in Mac OS X ...

http://support.apple.com/kb/HT2712

10.7 & 10.8…

System Preferences>Network, top of window>Locations>Edit Locations, little plus icon, give it a name.

10.5.x/10.6.x/10.7.x instructions...

System Preferences>Network, click on the little gear at the bottom next to the + & - icons, (unlock lock first if locked), choose Set Service Order.

The interface that connects to the Internet should be dragged to the top of the list.

10.4 instructions...

Is that Interface dragged to the top of Network>Show:>Network Port Configurations.

If using Wifi/Airport...

Instead of joining your Network from the list, click the WiFi icon at the top, and click join other network. Fill in everything as needed.

For 10.5/10.6/10.7/10.8, System Preferences>Network, unlock the lock if need be, highlight the Interface you use to connect to Internet, click on the advanced button, click on the DNS tab, click on the little plus icon, then add these numbers...

208.67.222.222

208.67.220.220

Click OK.

i think i know exactly what your problem is, do you keep your machine running all day and all night long? like 24/7? then that might be your problem, you have to download a ram clean tool from the app store or somewhere else (about 5 free ones that work out great so that wouldnt be a problem).

just make sure you run in once in a while and you should be good to go so. i had this problem too with my 4gb's of ram so i recently upgraded it with 2x8 stripes and now it runs 20gb's of ram and i dont have this problem any more, can keep my entire browser open with tabs with a length of an inch (27")

good luck with this solution !!,, oh and make sure you click this solved my question to get me some credits if you want, would be great =)

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regards, Coengi

These type of "cleaning tools" run on a Mac or through OS X are not a very good idea and can create more serious problems with a Mac and/or OS X.

What these programs claim to do causes other, more serious issues than they solve and these types of apps should be avoided.

They are " crapware"

However, you are correct about the OP installing more RAM into the iMac. The OP doesn't have to run 20 GBs of RAM or more  to solve the issues, though.

Any additonal RAM installed up to 16 GBs will be sufficient and I have already covered this in one of my posts to the OP.

This is an ANSWER and not a request for help.  I didn't know where to post this and apologize for stepping where I shouldn't. 

I have a Late 2012 iMac that has been crashing, running slow, and now worse;  all my documents and everything in Finder have disappeared.  I discovered a fix that may help others with similar issues.  I use my iMac for use with Logic Pro X and Mainstage where freezeups are a disaster.  I thought about this awhile and realized I had installed a new USB 3.0 external hard disk a few months ago when this issue began. I began to experience more and more crashes and extremely slow performance.  Beach-balls and slow performance were common.  But the last straw was when everything in Finder disappeared.  My Document and Application folders were empty.  If I clicked on "About This Mac" I saw nothing.    It took two tries to prove my theory, but here's what I did to make "the fix".  I unplugged the USB 3.0 drive and all USB and external devices.  I did a restore of OSX Mt Lion.  All was back to normal.  But after plugging in the USB 3.0 drive, all the afore mentioned problems came back.  I did one more restore, this time NOT connecting the USB 3.0 external hard disk.  Now I have gone some time and my iMac's speed is back to "incredible", no beach balls or any sign of slow performance in recording or running any other app.  Apparently, the 2012 iMac; although spec'ed to be USB 3.0 compatible, may not be all that compatible. Hope this helps someone!