Question: I've got an older iMac running macOs Mojave. It is terribly slow and I am interested to see if we can improve the performance. Can you recommend steps to consider to improve the performance? 

The performance issues I see impact the operating system and all applications. It's not specific to a single app.

When viewing the About modal:

* MacOS Mojave Version 10.14.6 (18G103)
* iMac 27-inch, Late 2012
* Processor: 3.2 Ghz Intel Core i5
* Memory: 8 GB 1600 MHz DDR3
* Graphics: NVIDIA GeForce GTX 675MX 1GB

Looking at the System Report, I see:

* Status on Memory Slots is OK. Has 2 4GB 1600 DDR3 cards.
* Hard Drive: I have about 500GB of available storage on 1TB hard drive

Other than these basic stats, I'm not sure what would be relevant to help diagnose the performance issues on this machine. What information could I provide to help determine the issue? Thank you.

Answer: 
There are basically two things you can do:

* Get more RAM. There are two free slots, so adding 2x4 GB is easy and having 16 GB instead of 8 GB will make a difference
* [Replace the drive](https://everymac.com/systems/apple/imac/imac-aluminum-tapered-edge-faq/how-to-upgrade-imac-hard-drive-aluminum-2012-2013.html) with an SSD

I have a late 2012 Mac mini as my main machine, with 16 GB and a Fusion drive. Definitively not the fastest machine but useable, even with big RAW files etc.
1. Replace the hard drive with an SSD. I recommend an SSD of at least 1 TB again. Not sure about if this is possible with your iMac either but I had my original 1 TB drive left inside my mid-2011 27” iMac. I am now happily using both drives. My iMac boots from the SSD which has the OS and the programs while my pics, videos and music are on the original drive. If keeping the original drive inside the computer is not possible, you can (have it) put it inside a proper enclosure and use it as an external drive.
2. Have the RAM increased to at least 16 GB and more if you can afford it.

Each of these two alone should make a big difference and together they should make you feel like you got a brand new computer. Plan ahead well about what to do, especially when adding / replacing the hard drive, and, if you are not already doing so, backup with Time Machine before doing or getting done either.