time to upgrade hard drive as imac to slow

Before you start replacing components check to see if there's any 3rd party software, i.e. "cleaning", "optimizing" or anti-virus software that might be contribution to the proiblem:

Download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Use the Add Text button to include the report in your reply:

Also assign Full Disk Access to Etrecheck so that it can get additional information from the Console and log files for the report:

Hi.

You can replace the mechanical hard drive for your iMac relatively easily.

If you are going to do this then you should opt for an SSD drive this time.

Your existing mechanical drive is the main reason that things are slow because it has a slow read/write speed that cannot cope with the size and demands of newer Mac OS operating system.

You must make sure that you get the correct SSD drive your device.

"About this Mac" will tell you the model and the serial number.

Consider buying your SSD form a company like Crucial. They have an excellent device search facility that will match your device with the correct SSD. They also have videos and step by step "how-to" guides.

I personally favour iFixit for buying screen replacement kits as their site is very clear and their products are good quality and arrive promptly. iFixit also have very clear "How-to" guides.

Try taking a look at ifixit

You have a late 2012 version.

https://www.ifixit.com/Device/iMac_Intel_21.5%22_EMC_2544

And here

https://guides.crucial.com/Guide/iMac+Intel+21.5-Inch+EMC+2544+SSD+Replacement/707

I personally would not recommend replacing the disk with a conventional mechanical hard drive as it is unlikely to be any fast than the current one. Crucial lets you look up your model and recommends a suitable SSD drive.

Please do not be concerned with the version. I STRONGLY recommend that you use OWC's. They supply a complete kit witch includes the drive you need, tools needed and also provide an instruction video. You can find the complete kit at:

https://eshop.macsales.com/shop/ssd/owc/imac-21.5-inch/2012

and the install video at:

https://eshop.macsales.com/installvideos/imac_21inch_late_2012_ssd/

While Crucial makes good products they do not provide complete instructions or kits. OWC specializes in Macs and have the knowledge to coach you along the way.

Please CAREFULLY review the video. If you decide this is too much for your skill level then contact an AASP in your area and ask them to install for you. Prior to doing the work it is also VERY wise to create a bootable clone on an EHD using SuperDuper! so you can easily format and restore from the clone.

I agree with Old Toad that before you jump right into replacing the hard drive, it would be better to understand what caused the issue in the first place. Simply replacing the hard drive and reinstalling your software from a Time Machine backup may simply reintroduce the problem if it was caused by software.

In my experience, performance issues like those that you describe can commonly be caused by software (malware or adware inadvertently installed), or a hard drive that is showing lots of reallocated sectors in the SMART (Self-Monitoring Reporting and Analysis Tool) logs that are tracked in hard drives. Once the error count beings to climb the performance will progressively get worse. Old Toad's suggestion to upload an Etresoft report will answer many of the questions. You could also use a tool to view the hard drive's SMART logs. One option is to use the demo mode for SMART Utility by Volitans software.

https://www.volitans-software.com/apps/smart-utility/

If you choose to open your iMac, be very careful. The power supply is exposed when the display is removed and can provide a dangerous shock if you accidentally leave the power cord plugged in.