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iMac slows entire wifi network

I have within the past week setup a brand new iMac and have all software up to date. Specs:

21.5-inch iMac with Retina 4K display

Configuration

3.1GHz quad-core Intel Core i5, Turbo Boost up to 3.6GHz

Intel Iris Pro Graphics 6200

16GB 1867MHz LPDDR3

1TB Fusion Drive


My typical wifi connection speed is 10 Mbps. After seemingly working fine for 2-3 days, whenever the iMac wifi is connected to the network the speed on all connected devices slows to about 1Mbps. After calling Apple support, the senior advisor I ended up talking to led me through a complex procedure of changing settings and turning the computer on and off with the result of getting the problem to go away. About 30 minutes later (the first chance I had to look again, during which time the computer was not being used) the problem was back. While reading the forums I saw someone suggest to another person that they might set up a new use profile to see if the problem they were having went away. Armed with this info, I logged into the Guest User account and the problem was not evident. Speeds for all devices, including the iMac were 10 Mbps as expected.


If I am logged into my original user account, I can run a speed test on another device, turn on the iMac wifi, and watch the download speed drop from 10 Mbps to 0.5 Mbps.


Has anyone seen this problem before? I am relatively new to Macs (I use them at work, but have never owned one) but if this was a Windows computer I would set up a new user account and delete this one. Is there any reason not to do this? I will continue to work with Apple, but was curious if anyone here knew anything about this.


If it is important, I am connecting to an Airport Extreme, and I would stress that the problem is very predictable and repeatable. Also, I do not know of any programs that are running in the background that might be using bandwidth. When I go to Force Quit, even when Finder is the app listed this problem occurs. I have Dropbox running in the background, but it does not seem to be the problem (and for what it's worth, there are no files that are syncing). Finally, when I look in Activity Monitor I see no network traffic, but the slowdown still occurs.


Thanks.


Message was edited by: jaycaustin

iMac (Retina 4K, 21.5-inch, Late 2015), OS X El Capitan (10.11.4), null

Posted on May 2, 2016 9:52 AM

Reply
9 replies

May 2, 2016 10:37 AM in response to jaycaustin

Might be corrupted network preferences.



Open the Finder. From the Finder menu bar click Go > Go to Folder


Type or copy paste the following:


/Library/Preferences/SystemConfiguration


Click Go then move all the files in the SystemConfiguration folder to the Trash.


Restart your Mac. See if that makes a difference.


Your Mac will generate a new SystemConfiguration folder for you.



If that doesn't help, a Safe Mode boot deletes system caches that may help.



Top left corner of your screen click the Apple > Shut Down.


After your Mac shuts down, wait 10 seconds, then press the power button.


Immediately after you hear the startup tone, hold down the Shift key. You should press the Shift key as soon as possible after you hear the startup tone, but not before.


Release the Shift key when you see the gray Apple logo and progress indicator.



Once you are in Safe Mode, go back to the Apple menu. From the drop down menu click: Restart





About Safe Mode

May 2, 2016 2:47 PM in response to Linc Davis

In recovery mode, the iMac's wifi status does not effect the rest of the network.


In between replies, I decided that based on my sucess with the Guest Account, I would try to create a new user account to see if it had the same issues. I can tentatively say that so far, the new User Account does not seem to effect the network. When I switched back to the original network after restarting from recovery mode, it seemed like it took a minute, but the network crapped out again. The Guest account and new User Accoutn continue to work as expected...

May 2, 2016 3:26 PM in response to jaycaustin

I will also now add that when I switched back to the User account, the problem did not show up immediately. So it is possible that I did not give it enough time in recovery mode. I am on the phone with Apple now so if they do not fix it I will try recovery mode again and wait longer between turning on the wifi and running the test. Yesterday though, the effect was immediate and today it seems like there is a delay...

iMac slows entire wifi network

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