wondering average lifespan for iMac - using for graphic design

In addition to Kappy's suggestions I suggest checking the following items:

1) A hard drive that is experiencing read/write issues can manifest itself as a slow system (pauses, beach balling). You can quickly check the SMART status of the drive using an app that can read the SMART logs from the drive. One option is Smart Utility from Volitans Software, and another option I learned about today by reading the comments from other users is an app called DriveDX. Demo copies can be downloaded from the respective developer websites.

2) Check for adware. This has been a source of frustration for my parents who are lured into clicking on links that they shouldn't. Tools like Etrecheck and MalwareBytes provide options for evaluating a system for common issues, and then provide options for resolving many of them.

There are lots of other questions that need to be answered in order to provide a more detailed answer including more information about the model of iMac, the amount of memory installed, and which version of Mac OS it is running. If you continue to have issues I suggest upload an Etrecheck report.

One word of advice is to always make sure you have at least one backup. It's a lot easier to recover from an issue when there is a backup available, especially if your livelihood depends on it.

If your iMac is running slow check what kind of hard disc you are using. If it's a convetional hard disc you may start want to upgrade it to a Solid State Disc. The price for a 1Tb it's about 200 dollars plus service, you would have to bargain what service provider charge on your. Believe me, booting it up with a solid state and more memory would give you a better performance that could last you for 4 more years.

Ways to Help Make a Slow Mac Faster

1. 17 Reasons Why Your Mac Runs Slower Than it Should
2. Slow Mac Performance? This Article Solves It!
3. Fix slow start-ups in OS X | MacFixIt - CNET Reviews
4. How to fix slow shutdown and startup times. | MacTip.net
5. 6 Easy Tips to Speed Up OS X Yosemite on Your Mac
6. OS X El Capitan- If your Mac runs slowly
7. Visit The XLab FAQs and read the FAQ on Performance
8. Tips to Fix Issues that affect Mac Performance
9. Avoid using any third-party cleanup software. Usually, this software does more bad than good. You don't need it. All computers become slower over time even under normal use. Experienced users erase the hard drive and do a clean install from scratch from time to time; or whenever installing a major OS upgrade. Doing so means you must maintain regular and multiple backups.
10. If you have enabled iCloud Disk and are storing your Documents and Data in iCloud, then consider turning that off and signing out of iCloud. It can slow down the computer considerably. Please see the following from the user, fotomac: "The solution was to SIGN OUT of iCloud and my problem STOPPED! NO MORE SPINNING BEACHBALL! My computer's speed increased to what it should be and all my Apps now work!"
11. Add more RAM or cut back on the number of concurrently running applications and utilities. Remove unnecessary anti-malware software and any software that promises to clean your Mac. Check for runaway processes: Runaway applications can shorten battery runtime, affect performance, and increase heat and fan activity. Also, visit The XLab FAQs and read the FAQ on dealing with The Spinning Beach Ball of Death.
12. The Ultimate Fix: Backup everything, erase the drive, reinstall OS X, and restore your data from the backup. Reinstall third-party software from original media/scratch.

Continued from above:

14. If none of the above helps then see How to Downgrade macOS High Sierra and macOS Reversion- 

      How to Downgrade from High Sierra.

15. If you get here without success then it's time to make an appointment at the Apple Genius Bar for 

      service. If you need to find an Apple Store - Find a Store - Apple.

I am using a 2013 Macbook air that is still running fine. I even spilled orange juice on it 3 or so years ago. 8^)

The computer does not get slower than when it was new. What slows it down is the stuff you put on the computer.

A Troubleshooting Procedure that may Fix Problems with macOS El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

    Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Disconnect all third-party peripherals.
3. Resetting your Mac's PRAM and NVRAM
4. Reset the System Management Controller (SMC)
5. Reset your Startup Disk and Sound preferences.
6. Start the computer in Safe Mode. Test in safe mode to see if the problem persists, then restart normally.
7. Use Apple Hardware Test to see if there is any hardware malfunction.
8. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list.  Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
9. Repair permissions on the Home folder: Resolve issues caused by changing the permissions of items in your home folder.
10. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
11. Download and install the OS X El Capitan 10.11.6 Combo Update or 10.12.6 Combo Update or Download macOS High Sierra 10.13.5 Combo Update or Get MacOS Mojave now from the Mac App Store as needed.
12. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
13. Erase and Install OS X Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

     1. Select Disk Utility from the Utility Menu and click on the Continue button.

     2. When Disk Utility loads select the drive (out-dented entry) from the Device list.

     3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.

     4. Set the Format type to APFS (for SSDs only) or Mac OS Extended (Journaled.)

     5. Click on the Apply button, then wait for the Done button to activate and click on it.

     6. Quit Disk Utility and return to the Utility Menu.

     7. Select Reinstall OS X and click on the Continue button.