when should I add more memory to my iMac?

You need to learn how to use Activity Monitor in order to learn how to manage your the RAM in the computer and then discern whether you should upgrade to more RAM. Please refer to Activity Monitor to read System Memory and determine how much RAM is being used to get started.

If you decide it is time to upgrade the RAM please ONLY purchase macsales.com RAM, why because iMacs are VERY sensitive to using quality RAM and macsales.com only provides high quality reasonably priced products.

You may also find that your computer is slowing due to what has been installed on it. If you have ANY of the following types of third party apps installed, these are at best worthless and at worst can harm Mac OS and it's performance:

• Antivirus
• Cleaning
• Security
• VPN
• Maintenance

If you do have any of these types of apps installed, then the first thing to do is to navigate to the developers web site and learn how to correctly uninstall and then execute those instructions. Then restart in Safe Mode and restart normally and test. If the problem persists then please let us know for further instruction.

My 2020 iMac is running big apps really slowly ...

Few "slow iMac" reports here turn out to be caused by RAM starvation. Usually it is storage—mech hard drives— but the 2020 iMac 27 shipped with flash storage. So something else must be at play. Adding RAM seldom makes a noticeable speed gain that the user senses in use.

There are a couple of things where Flash storage can slow, but they are rare. Because we know so little about your computer's specs and software loadout, your posting a system config report is about the only effective way we can effectively help here. Fortunately, there as a safe way to do that.

If you wish a data-driven evaluation in this setting where we can neither see nor touch your computer, please post an EtreCheck report. We can quickly and within the limitations of these forums help you determine what issues are at play without our playing a protracted game of "20 Questions" with you that could go on for days. 

EtreCheck Pro is available here: https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

From one posting, we can see hard data about drive performance, software issues and interferences, and RAM usage. Etrecheck is the development of a long-serving and trusted ASC contributor. It is a reporting app, not a "fix-it” app, expressly for displaying information in these forums to help us help you remotely. It will not reveal any personal or secure information.

Please see this excellent user tip on how to post long text reports like EtreCheck's into a forum response:

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

Please post the entire report. What seems insignificant to a new EtreCheck user can speak volumes to those of us who have reviewed thousands of those reports.  

>> About 510 GB left on my hard drive, which is the main drive

if you indeed have a factory 1TB internal SSD soldered to your main board -- that is not the problem

that's different than a SATA SSD (which I'm not sure a 2020 5k iMac can contain internally, if it has the connection)

you could also run a macOS utility like DriveDX to note the state of your SSD health percentages -- and flagged failing issues

you may want to make sure your SSD is not encrypted as that can slow things down unless you need that encryption -- I am not sure if the T2 chip like the 2020 has -- encrypts by default or if it allows you to unencrypt the drive

>> Currently have factory memory: 8 GB 2667 MHz DDR 4

that machine is RAM starved and most likely the issue, in my opinion, add 32GB (16GBx2) to your two empty slots (OWC is $61.99 new) -- total 40GB -- and you should feel a significant boot in performance running the more memory intensive apps

OWC has great videos on upgrading the ram in that machine -- the small back door opens and you simply plug in the new cards if memory serves me (unplug the machine before you begin)...

Dropbox, Chrome, all Microsoft apps load very slowly.

That screams "storage problem," not RAM starvation.

Summarizing instead of posting the EtreCheck report denies us the two data sections most pertinent to your question. We still do not know the storage type, nor its performance. Because storage issues are about 50x more likely to cause slow iMacs than insufficient RAM, we can't help effectively here without drive data.

The Fusion drive that came as standard storage in that iMac model is, based on long history here, the most likely suspect. You would be wasting money to buy RAM without knowing if you instead have a storage problem. More RAM cannot fix underperforming storage.

Please post the entire report. Its developer made it so it will not give away any private information.

Please read the following user tip and restate your question/problem in detail: 

     Writing an Effective Apple Support Communities Question - Apple Community

What model and year is your iMac?

What system version are you running?

How much free space do you have on your boot drive?

What apps are you having problems with?

How much memory do you currently have installed?

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your iMac?

When you're running one of the problem apps have you checked the Memory Pressure graph in the Activity Monitor app to see if it's in the green or yellow region?

Are you experiencing the slowness only when running those apps? If not describe when the slowness is obvious.

Okay thanks for the 411 about your system. Please refer to the Activity Monitor link and determine if you need additional RAM. Adding additional RAM will NOT increase performance though, so please be aware of that.

You say that you have a 2019 27" iMac. Those shipped with two different 1 TB storage options:

• 1 TB Fusion Drives. These combined a 1 TB hard drive with a small, skimpy amount of SSD storage. (Nothing like the 128 GB included in the original 1 TB Fusion Drives.)
• 1 TB flash storage, i.e., 1 TB SSDs.

If you have a 1 TB Fusion Drive, getting an external SSD and making it your startup drive might make the system start up more quickly and launch applications more quickly.