Q: Worst update ever - my computer loads slowly, is incredibly sluggish, and totally unusable.

Hi Peter and thanks for your immediate response. Sometimes the work time chases us and difficulties as YOSEMITE has caused , are not welcome. Of course I don't shout. But as Jaypeedee said, I don't shout. I an really disappointed since I have just moved from windows to mac and the result after  a week is to try to find another Windows pc to complete my work. Also, When I clicked on upgrade to Yosemite I had not kept any back up. This is the situation. What do you suggest me now to do? Keep my files and format? IS there a way to restore in Mavericks or I have to keep my files and format? If so, a)from where shall i download Mavericks? b)can I install iOS Mavericks on Mac partition without any affect to windows partition or i have to do all from the beginning which means again partition again windows installation and Mavericks?

Have you checked Activity Monitor to see which processes are using your processor?  That's a very good way to see what's going on.  There may be just one or 2 processes that are the problem.

I've been running Yosemite on my mid-2012 MBP without any problems.  It seems even faster than Mavericks. 

Lena, typing in ALL CAPS is considered shouting on the Internet.

Clearly either Yosemite didn't install correctly, or you might have some conflicting kernel extensions.

I'm on an early 2009 iMac and Yosemite runs faster than Mavericks did. You didn't mention how much RAM your machine has.

I would make a backup of your Mac (you do have a backup drive, right? You really should, and the new TimeMachine works great), boot up from the recovery partition, and reinstall Yosemite.  If you really want it to run great, erase the partition you are installing Yosemite on, do a clean install, and then restore your files from your backup.

I have also found that  when the hard drive gets very fragmented performance will suffer.

Download and run Etrecheck.  Copy and paste the results into your reply. It's a diagnostic tool that was developed by one of the most respected users here in the ASC.

For those having problems with performance, please, read the following guide, it cold help you:

                 Guide: How to solve Yosemite memory leaks and CPU usage            

Hello astone8122, Thanks for your advice. Yes I have checked Activity Monitor but I cannot recognise problems. The only problem seems to be in Google Chrome Helper (which is red). I look forward to see  my iMAC work faster with Yosemite  than with Mavericks and not return to it.

Hello David, Thank you for your detailed response. At least I am trying to fix the problem not being alone! So what I have done is to click on Yosemite upgrade. It was working with a normal way during upgrading As it concerns kernel extensions...I have no idea for it :-( The some details for my mac isQ I bought it in 2012, it has 4GB RAM and 1 TB HD (with 500 GB Mac and 500GB Windows. I have already done back up my files in an external HD and... you suggest me to reinstall Yosemite I will try...I will give one more opportunity to this OS Could you please tell me the steps for your advise: " If you really want it to run great, erase the partition you are installing Yosemite on, do a clean install, and then restore your files from your backup". Thanks again

Activity Monitor - Mavericks  also Yosemite

Activity Monitor in Mavericks has significant changes

Performance Guide

Why is my computer slow

Why your Mac runs slower than it should

Slow Mac After Mavericks

Things you can do to resolve slowdowns  see post by Kappy

Did you try deleting Chrome for now?  If it's chewing up a lot of CPU, I'd try that.  If that helps, try reinstalling it.

Amy