iMac (Retina 4K, 21.5-inch, Late 2015) with 8GB Memory

If you wanted to add more Memory because the system responsiveness is poor, then I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as shown here: How to use the Add Text Feature When Post… - Apple Community. We will then examine the report for issues in your Mac.

Jack

The RAM in the Retina 4K, Late 2015 21.5" iMac is not upgradable.

see> Install memory in an iMac - Apple Support

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior (much like you are experiencing) and make Mac OS appear buggy. 

Please locate the developers uninstall instructions for MacKeeper and follow to the letter. Then restart in Safe Mode per the directions in Use safe mode to isolate issues with your Mac  and then restart normally. 

Then run a fresh EtreCheck report and look through it to verify there is ZERO mentioned of MacKeeper. 

Next, is there is no evidence that  your computer is backed up using Time Machine or any other method. If this is correct that is very short sighted and a 100% guarantee you will lose data, likely all of the data stored on the computer. Operating ANY computer without a backup is no responsible unless you simply do not care about your data and apps. If you do care about your data,  your #1item is to begin backing up using Time Machine (Back up your Mac with Time Machine - Apple Support) which requires and external HD, many experienced users on these forums prefer the OWC Mercury Elite Pro (OWC Mercury Elite Pro) due to it’s high reliability rate, reasonable cost and OWC’s service. 

Good luck!

Another big factor:

Hardware Information:

iMac (21.5-inch, Late 2015)

iMac Model: iMac16,1

1.6 GHz Dual-Core Intel Core i5 (i5-5250U) CPU: 2-core

You have the crippled educational/institutional iMac with a slow laptop-class processor. The standard consumer iMac (21.5-inch 4k) for 2015 had a desktop class quad-core processor, and the faster quad processor was an option in your iMac:

That, coupled with a under-spec internal hardware Apple opted to install, are why it is slow. Ridding it of MacKeeper may help your drive scores. However, that model drive cannot post read/write scores higher than about 70-80MB sec. If your MacBook Air has a solid state drive, it could be doing over 2000MB/sec, making it "feel" much faster with faster booting and app load times.

Stevenhals85 wrote:

I had deleted that completely last night. I also ran in safe mode and did everything appropriately that I could. I have an external hard drive I used previously to this so all things are back up.

I just don't understand why it just runs slow considering there is a huge difference in space and age from this mac from my macair and my air runs faster than this.

What you have is the slowest Mac that Apple offered at the time of purchase; 8 GB memory and a 5400 rpm hard drive. It is good for simple web surfing, emails, texting and simple text documents. And then to top if off you added one of the worst offenders to Mac performance, MacKeeper.

I had deleted that completely last night. I also ran in safe mode and did everything appropriately that I could. I have an external hard drive I used previously to this so all things are back up.

I just don't understand why it just runs slow considering there is a huge difference in space and age from this mac from my macair and my air runs faster than this.

i am not the original person to this post. I found this post due to me having load issues being slow. My macair laptop is not able to use this Monterey OS as it is not compatible anymore. Which still runs faster than this iMac desktop that is running the updated version. I also had to factory reset this desktop as it was not running correctly at all, which in return made me question why this desktop has more memory than the laptop and why it runs slower. I was wondering if there was a way to make this run faster.

EtreCheckPro version: 6.6 (66014)

Report generated: 2022-06-12 01:25:05

Download EtreCheckPro from https://etrecheck.com

Runtime: 6:17

Performance: Below Average

Guys,

Did anyone notice the the person posting the report is not the OP?

Stephen, can you tell us in you are the same as the Original Poster GrahamDay?