Yosemite is slow on a mid 2011 iMac i7 2600

I have it running on the same machine here, and it works just fine. So the issue is yours, not Yosemite. I suggest you backup, erase your drive, reinstall Yosemite on a clean drive.

Did you update your Mac from Mavericks or did a clean install?? I have a mid-2010 iMac, did a clean install of Yosemite and works fine.

I also have a 2011 27" iMac with a SSD and internal HD running Yosemite, the GPU is also the AMD Radeon HD 67770M, my machine's performance is perfect.. The problem is specific to your machine and not Yosemite.

Launch the Console application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Console in the icon grid.

The title of the Console window should be All Messages. If it isn't, select

SYSTEM LOG QUERIES ▹ All Messages

from the log list on the left. If you don't see that list, select

View ▹ Show Log List

from the menu bar at the top of the screen. Click the Clear Display icon in the toolbar. Then take one of the actions that you're having trouble with. Select any messages that appear in the Console window. Copy them to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.

The log contains a vast amount of information, almost all of which is irrelevant to solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.

Please don't indiscriminately dump thousands of lines from the log into this discussion.

Please don't post screenshots of log messages—post the text.

Some private information, such as your name, may appear in the log. Anonymize before posting.

There seems to be a problem specifically with this iMac model (mid-2011) and Yosemite. My machine was all around slower after upgrading until I found this article from Parallels describing the problem. I ran the trace_intr tool from this page and my machine was indeed having too many interrupts from

com.apple.driver.AppleACPIPlatform.

The solution for me was to do the following:

1) Open a terminal

2) Enter and run the following command, entering your password when prompted:

sudo nvram boot-args="debug=0xd4e"

3) Restart the computer

After restarting, there was an obvious difference in terms of UI responsiveness all around. I also verified using trace_intr that the interrupts were no longer happening.

I'm no expert on OS X internals, but I assume this kernel parameter enables a debugging mode that disables the ACPI interrupts that cause this behaviour.

This worked perfectly, or at least perfectly enough. The trace program identified the same issue: millions of interrupts coming from the ACPI driver. Performance is much better now. Not Mavericks-like, but much better. I can even quantify it with (the albeit a bit old) Xbench:

Hardware: Mid 2011 iMac, 2.7 GHz Core i5 w/ 16 GB RAM, Radeon 6770M w/ 512 MB RAM, 1 TB SATA HDD

Mavericks overall score: 228.26

Yosemite pre-nvram fix overall score: 82.42

Yosemite post-nvram fix overall score: 117.89

Yosemite showed a massive hit to CPU, RAM, 2D GPU, and 3D GPU performance. After the above fix was applied, the CPU and RAM performance hits disappeared, and the 2D and 3D performance recovered about half way to their original scores. Reducing transparency has almost no effect.

A coworkers 2010 13" MacBook Pro showed no ACPI driver issues and no improvement with the additional boot parameter added, so I can confirm that this fix looks to be specific to the 2011 iMacs (which have certainly had more than their fair share of issues). It seems with pre-2012 laptops, usable performance with a 2nd display connected is a thing of the past, as they seem to be fine with just the built-in display.

It's now up to Apple to fix their ACPI driver for those out there uncomfortable with the command line, and hopefully optimize their graphics drivers further in future updates. My 2008 MacBook Pro has a lot of life left in it...or did until I put Yosemite on it. I've reverted to Mavericks on these 2 machines once already, and had hoped 10.10.1 would do the trick, but it hasn't.

Send Apple feedback. They won't answer, but at least will know there is a problem. If enough people send feedback, it may get the problem solved sooner.

Feedback

Done and done. I encourage everyone experiencing these issues to do so. QA at Apple seems to have gone downhill, with only the latest devices tested against new software. Look at what iOS 8 did to A5-based devices (and 8.1.1 barely addressed), one of which is still being sold!

I've also submitted feedback and I encourage others to do the same!

https://www.apple.com/feedback/imac.html

All of this chimes with my experience on my fully spec'd 2011 iMac. I've tried a fresh install and upgrade of 10.10.1 and there's no difference. I'm running Mavericks again and the world is good.

This problem is especially painful with Virtual Machines on Parallels. Here's a little more supporting info from the Parallels Knowledge Base:

http://kb.parallels.com/en/122767

Not only did the solution from danielpgross solve my slow iMac problems, it also solved an issue I have had for nearly two years with my screen/system not going to sleep after being inactive. SO excited about this!

Thanks!

Thank you Kappy,

how do I do that? is there information available to guide me?

as you can see, I'm not an expert in the matter.

I do have a full back up with a Time Capsule.

Any advice is appreciatted

Thanks

Diego

I almost bought a new Mac until I saw this... My Mac is much faster as before after applying this... I totally don't understand this hasn't been fixed by Apple by default...