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Please run an EtreCheck Report. Please click http://www.etrecheck.com/ where you will find the app. Once you downloaded the free app and created the report please post it as your reply to this post. In order to post it, please use the following link for more detailed instructions on how to correctly post the results:

https://discussions.apple.com/docs/DOC-250000211

We can then look for obvious issues that may be causing your problem.

I'd suspect at least two causes just guessing since you don't offer much information:

1- too little memory

2- iMac is older model and the hard drive is getting old

You have a number of things going on that need to be addressed.

First, you have CleanMyMac installed, this is considered malware by experienced users. Mac OS does not benefit from or require ANY third party maintenance apps including cleaning apps, antivirus or anything else. Please locate the developers instructions to uninstall CMM and get rid of it completely!

Next, you have adware installed. This must be removed as it hinders performance. Use the EtreCheck report to uninstall this.

Finally, your 2011, 27" iMac is starved for RAM. It has 4GB of RAM which was fine 8 years ago when it was running Snow Leopard! Current versions of Mac OS need at least 8GB of RAM to run well, I would recommend upgrading to 12 or 16GB of RAM. Please only use Crucial.com or www.macsales.com RAM, the reason for these two vendors is iMacs are sensitive to RAM and both vendors supply high quality products at reasonable prices. Installing RAM in a 27" iMac is a 5 minute process.