2019 Intel iMac running Sequoia is very slow at start up.

is your Mac HDD based? if yes, that explains the slowness. HDDs are very slow compared to SSD based Macs. especially when running Big Sur and later. the easiest way to speed things up would be to use an external SSD with sequoia installed. to implement that, please see Use an external SSD as your startup disk with your iMac or Mac mini - User Tip

but if the Mac is already SSD based, i'm thinking you should download and run the free version of EtreCheck so we can see if you have some software installed that is causing your issue. make sure you give "full disk access" to etrecheck. Learn how to use it by reading Using EtreCheck. if you need help interpreting the report, you can see how to post the report here by reading How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report. and it automatically obscures sensitive things (like serial numbers) so you don't have to worry about sharing the report here.

Please determine if the same problems occur in "Safe Mode": How to use safe mode on your Mac - Apple Support.

Starting a Mac in "Safe Mode" will take a long time. That's normal. Wait for it to finish, then shut down the Mac using  > Shut Down... To advance troubleshooting I suggest you de-select "Reopen windows when logging back in".

Wait for it to shut down, and then start it again in the usual manner.

Any change?

Was there a reason you shut it down to begin with? What is that reason?

To add to the wise contributions of my colleagues, yes, EtreCheck is the best tool for our identifying issues in a computer we can neither see nor touch.

However, there is a quick quiz:

• Is the computer slow to start up (cold boot)?
• Are big apps slow to launch?
• Once launched, do those apps seem to run at acceptable speed?

Three "yes" answers pretty much confirm you have the glacially slow and under-spec mech hard drive that Apple elected to install as the base storage in 21.5-inch iMacs made from 2012 through 2019. Adding RAM (a) is not practical in the model, and (b) won't help an underperforming hard drive anyway.

The EtreCheck report will turn suspicion into hard data, and can reveal more, for example, an optional Fusion drive that is misbehaving, or interference from unneeded fearware like anti-virus or client-side VPNs.

+1, please provide an EtreCheck report and we can look for obvious issues. Please remember, HD based systems use slower mechanical components than the newer SSD based Macs. This can translate into slower startup speeds as the computer ages as the mechanical components age and wear. When you are ready for a new Mac it will be considerably faster than your current machine.