speed up old iMac

Hmm, your post is short on details but... 21.5" Mid-2014 with the default 1TB HDD?

Yah, those don't play nice with the new APFS Apple file system. You will need to reinstall MacOS onto an external USB 3.0 SSD (or upgrade the internal drive to an SSD, but that probably isn't worth the effort).

Almost any SSD will do, but otherwise see this excellent post for instructions: Use an external SSD as your startup disk … - Apple Community

+1 for posting Etrecheck to look for other issues, especially if I'm guessing wrong and your iMac has a Fusion drive or SSD.

.. I ran the etrecheck software and it said the hard drive was slow..

Please post the report. There are often helpful but subtle clues therein that may not be obvious to the new EtreCheck user, but jump out at those of use here who have reviewed thousands of them. Sometime simply remove unneeded software helps.

Also, it you already have an Apple Fusion (hybrid) drive that is slow, those are a sometimes fixable at home with some commands. A functional Fusion drive is faster than the external SSD solutions appropriate to a 2014 iMac.

This user tip tells how to post the report here without garbling it. Remember Etrecheck scrubs all personal and sensitive security information before creating the report. It's safe.

How to use the Add Text Feature When Post… - Apple Community 

Install EtreCheck and post the report. Allow Full Disk Access.

https://etrecheck.com/

Use the Additional Text icon to paste in the report.

A 2014 is definitely obsolete meaning parts are difficult to obtain and it will not run current versions of Mac OS. If you are considering the external SSD approach (I wouldn't!) please refer to How to setup an external SSD as your startup disk. For an external SSD I'd recommend a https://eshop.macsales.com/shop/owc-mercury-elite-pro-mini/usb-c and DO NOT get the Bus powered version. Get one with its own power supply.

If you decide to get that new iMac after Xmas please read the following for your migration from the old computer to the new one. Also, you may also be a good candidate for a Mac mini. You will only need an external display and can re-use your old Mac's keyboard and mouse with the MM.

When you buy a new Mac the easiest, quickest and most reliable method to migrate from the old Mac to the new Mac is to simply connect the old Mac's Time Machine External Hard Disk to the new Mac using a USB cable before you turn on the new Mac. You will likely need a USB A to USB C adapter, you can find them on Amazon for $10 or less. When you start the new Mac for the first time it will startup Setup Assistant (SA), SA will get to a point where it asks if you are migrating from another computer, select Yes  and follow the on-screen prompts and it's done! It is THAT simple.

If you have already done SA on the new computer then simply connect the Time Machine External Hard Disk using a USB cable and launch Migration Assistant (MA) which is located in Applications - Utilities - MA. Then follow the on-screen prompts. 

The ONLY difference between MA and SA is MA will create a new user account that you need to log into.

Both SA and MA migrate applications (not 32 bit apps), data files & settings. They do not migrate Mac OS itself. Some professional apps such as Adobe and Microsoft apps may require re-installing their apps as they will not allow them to be migrated. As long as you kept the security keys for installing their apps, it is not an issue. 

Finally, never EVER use Wi-Fi when doing Setup Assistant or Migration Assistant. Why, it simply fails in most cases. Please read and follow the directions in Setup Assistant and Migration Assistant. If you do not have a Time Machine backup of the older Mac then create one! If you are not aware of Time Machine, please read Back up your Mac with Time Machine and finally if you need an External Hard Disk to use as a Time Machine External Hard Disk then many experience users on these forums prefer the OWC Mercury Elite Pro due to it's reasonable price, high level of quality and ease of use.