Dead Slow startup (iMAC late 2015 running Monterey 12.7.5)

These two entries tell the tale:

Drives:

disk0 - APPLE HDD ST1000LM024 1.00 TB (Mechanical - 5400 RPM)

Internal SATA 3 Gigabit Serial ATA

and

Performance:

System Load: 2.58 (1 min ago) 2.41 (5 min ago) 2.07 (15 min ago)

Nominal I/O usage: 0.88 MB/s

File system: 25.94 seconds

Write speed: 54 MB/s

Read speed: 66 MB/s

Unless you up-order a better storage optional than the slow and underspec mech hard drive, 21.5-inch iMacs made between 2012 and 2019 are all slow with current macOS versions. I have data from Macs I own showing the base mech hard drive in the 2011 iMac 21.5 is 30% faster than the equivalent drive in the 2012-2019 models.

Even healthy, the 5400 rpm drive that is rated a slow SATA 3G attached to a fast SATA 6GB bus can do no faster than 70-80 MB/sec. Our 2011 iMac 21.5 does 110MB/sec.

It is possible that removing the cleanup and anti-virus fearware will speed the drive somewhat but it is maxed at 80MB/sec regardless of what your remove. And 80MB/sec is glacially slow today.

You can take advantage of the external USB-3/solid-state boot drive option. In its least expensive incarnation it will do 400MB/sec all day and requires no professional installation:

Use an external SSD as your startup disk … - Apple Community

You can do this for less than US$100 depnding on the size of the SSD.

I'm just reinforcing everything you've already been recommended to do with the following:

First, 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 documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

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.  

Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint. 

Therefore I recommend you uninstalling, according to the developers' instructions, the following apps/software: Antivirus One, Cleaner Pro and ProtonVPN.

You can post the report to this site. To learn how to do that, please read How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report - Apple Community.

To learn how to remedy low storage, read Free up storage space - Apple Support.

Delete this file, restart & see if John Galt has more info/help.

~/Library/LaunchAgents/ProtonMail Bridge.plist ~/Library/LaunchAgents/ProtonMail Bridge.plist

That Mac's startup disk has plenty of free space so that's not the problem. The reason your Mac is slow is that its performance is not very good. Having installed and used at least a couple of ill-advised "cleaning" products are likely to have contributed to its demise.

Non-Apple "cleaners", "doctor cleaners", "memory cleaners", "doctor helper cleaners" etc are all scams that wreck perfectly good Macs. Uninstall those things. Rule 1 of Macs is don't install junk.

Still hoping John Galt will return to help, but...

You can use Drive DX to possibly get a better view of Drive health…

https://binaryfruit.com/drivedx

I follow this rule as well. I choose what whatever *help programs* only from Apple store.

That doesn't really matter. Most of the apps on the App Store are junk.

Currently I use the CLEANER ONE Pro .

Case in point.