I agree with the other suggestions here, adding more ram is easy but if you're already at 16 or sometimes even 8gb you won't notice much difference.

The biggest improvement you can make is putting in a solid state drive (SSD). If your hard drive is currently an old-school spinning disk putting in an SSD will make it feel like a new computer. I would recommend the following:

1. Familiarize yourself with the repair guides for your specific model at iFixIt.
2. Buy the SSD itself; Either a Samsung Evo 850 or a Crucial MX300 if the Samsung is too expensive.
3. Depending on your mac, you might need an internal mount for the SSD that takes up the same space as a traditional 3.5" drive. Like this Newertech AdaptaDrive
4. Because iMac drives have internal temperature sensors, if you just plop a random drive in there it won't report temperature and the Mac will respond by turning up the fans to 100%. You can solve this with software like SSD fan control or my preference is to use a replacement cable that includes a sticky-able temp sensor. You can buy the cable itself or the cable and a handy installation kit that will make getting in to your mac easier.

I'm happy to help with more details or assistance if you go down the path of an SSD upgrade. Good Luck!