when to upgrade my 2015 iMac?

In no particular order, please determine if the same problems occur in "Safe Mode": Start up your Mac in safe mode - Apple Support.

Use it long enough to determine if it's performing any differently while in that mode. If it runs just as poorly in Safe Mode the likelihood of hardware failure — usually a hard disk drive that isn't working properly — is the cause. They can be replaced. Either way, restart your Mac normally (not "Safe Mode") and observe again. Report your observations.

If it performs better in Safe Mode then the cause is likely software-related, which can usually be fixed without having to invest any money in a Mac that's getting a bit long in the tooth.

For a more in-depth examination and report of your system consider using EtreCheck and posting its report in a reply to this Discussion. Instructions: How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report - Apple Community

pipefighter wrote:

Is there a certain way to know when its upgrade time? It is running BigSur and will not update.

In my experience, and opinion there is no "certain" way to know. The decision was made for me when the developer of one of my primary applications announced that newer versions of their application would no longer be supported using Big Sur.

when to upgrade my 2015 iMac?

When it no longer does what you need it to do. It's a very personal decision that no one else can make for you.

Age itself does not cause a Mac to run slow. If you are interested in fixing yours, let us know.