iMac Very slow to boot up

Here's another item to consider. There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

That being said uninstall Trusteer according to the developer's instructions;

If you decide to get an SSD to use let me suggest getting an external SSD. That would be far less expensive than paying for labor and party to replace the internal HD with an SSD. And once you're ready to upgrade your Mac you can easily take it with you. I also wholeheartedly agree with rkaufmann87 on the OWC (MacSales.com) units. Call OWC's customer support and see what they recommend to meet your needs and budget.

Is your iMac also slow at loading web pages or just loading e-mail?

Try starting up in Safe Mode and test Mail. Then restart normally and re-test Mail.

see > How to use safe mode on your Mac - Apple Support

If the issue persist after restarting normally? Then I would suspect that you have installed some third-party AV-ware, Cleaning-ware or Optimizing Application that is causing the problem.

If the problem continues after den.thed's suggestions are executed please do the following:

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community

You problem is you purchased a base model iMac whose primary bottleneck is the 5400 RPM HD which is glacially slow. Also the computer is now obsolete, meaning Monterey 12.5 is as far is it will go. If it were my computer, I would replace it with a new 24" iMac with 16GB of RAM and a 1TB SSD rather than spending any time or money on your current machine.

However if you want to keep the old computer and improve it's performance, the ONLY thing that will work is to take the 5400 RPM HD out of the equation by adding an external SSD and cloning all the data on the HD to the SSD using either SuperDuper! or Carbon Copy Cloner. The external SSD I'd buy is either:

1. OWC Mercury On The Go Pro and a OWC On the Pro Go Power Adapter OR
2. OWC 1.0TB Mercury Extreme Pro 6G SSD and a SATA to USB adapter

My wife was using the same Mac model as yours, a late 2015 iMac with an internal HDD (mechanical drive). With Monterey it was close to unusable. We replaced her internal mechanical drive with a Samsung SSD (internal drive) and it has increased in speed by more than 10x (as measured by boot time, time to open applications, load web pages, etc.). It boots up in seconds instead of minutes. It used to take one to several minutes just to shut down, now it shuts down in 4 seconds. Browsers, software like MS-Word, etc. all open in a second or two now.

I am using this rejuvenated 2015 iMac myself now, and we got her a new M1 iMac, which performs spectacularly compared to what she was used to. But the 2015 iMac now is very peppy. The problem with the 2015 iMac is that it won't be able to run the next version of the MacOS after Monterey, Monterey is as far as it can go. But with an SSD boot drive (internal or external) you will get another several years of good use. The cost of replacing the internal drive with a 1 TB SSD (internal) was under $200, which includes the cost of the internal SSD plus labor by an Apple Authorized Service Provider to swap out the drives. I wanted the internal SSD for booting, not an external SSD. If you go with an external SSD, you would save about $50 over what I paid for the internal drive replacement. Also, a smaller capacity SSD is another way to save a little money if you go this route. I wanted the full 1 TB so I would not have to worry about disk space.

A new M1 iMac 24" currently costs between $1300 (low end) and $2000 (high end). So a peppy iMac, even if 2015, is much less expensive at ~ $200 for the upgrade. You do have options here.

One last thing -- your iMac's slowness sounds much worse than what we saw when we had our original HDD. Whatever option you pick, I suggest you migrate only your user files to the new drive, then reinstall your software fresh. Also, you have Trusteer security software, is that really necessary? Is it compatible with Monterey?

A few more items:

When you go to the Apple Authorized Service Provider, ask them to run diagnostics on the hardware to make sure the rest of the hardware is ok, as it is 7 years old and things do start to wear out. That way you won't go through the expense of the new internal drive replacement and find out something else is wrong. By the way, because of the age of the computer, I don't think Apple Stores will provide service for it, but the independent Apple Authorized Service Providers will.

One reason I like this 2015 iMac is that it has three USB-3 ports (which are fast, 5 Gb/s) and an ethernet port (1 Gb/s) and 802.11ac WiFi which means you also have fast networking. It also has two Thunderbolt 2 ports, which admittedly are a bit dated but there are adaptors and docks one can still obtain that allow other peripherals to be connected to those as well. It also has an SDXC card slot. So it actually has more ports than some of the brand new Macs. I am a little sad that it will not (according to Apple's specs) run the new upcoming Ventura MacOS but equipped as it is, it is quite capable for the next few years at least running Monterey (with an SSD instead of the HDD).