iMac 2015 slow boot and program performance

You don't need more RAM – 32GB is likely plenty. You can check by looking at Memory Pressure in Activity Monitor. If it remains in the green then you're okay.

You don't need an external bootable drive. Unless I'm misreading this you appear to have about 780GB available on a 2TB startup drive. It is a 7200RPM HDD, so you could run a drive diagnostic like DriveDx to get a better look at the drive health. In fact, you should run DriveDx on all of your drives, internal and external, as it's possible that any of them could be experiencing some hardware issue that contributes to poor performance.

You do need to get a handle on all of the software that has been loaded into the iMac.

You have over 600 third party apps installed. With those apps are all manner of extensions and ancillary files.

You have CleanMyMac installed - completely unnecessary and often implicated in poor performance of Macs. A definite red flag!

You have IPVanish VPN installed - VPNs are less secure than you might expect and should only be used if required. Another red flag!

You have Parallels installed - do you run another OS in a virtual machine? This could compromise the Mac's performance if improperly configured.

You have both Google Drive and Dropbox running - each probably actively syncing files and keeping the drive(s) busy.

If this were my Mac and I was having these issues, I would seriously consider wiping the drive and starting over with a fresh install of macOS. I would evaluate all of the software that I have and reinstall only the apps that I regularly use. I suspect you may have more than a handful of apps that you don't use regularly, if ever.

I would also reconsider my use of Parallels and whatever guest OS you have installed. Do you really need it? Is it something that can be installed on another external boot drive? Just thinking out loud, as it were.

There's a lot going on in that machine.

It is your drive. This section of the report:

Drives:

disk0 - APPLE SSD SM0128G 121.33 GB (Solid State - TRIM: Yes)

Internal PCI 8.0 GT/s x4 Serial ATA

disk0s1 - EFI (MS-DOS FAT32) [EFI] 210 MB

disk0s2 [APFS Container] 121.12 GB

disk2 [APFS Virtual drive] 121.12 GB (Shared by 5 volumes)

disk2s2 - Preboot (APFS) [APFS Preboot] (24 MB used)

disk2s3 - Recovery (APFS) [Recovery] (519 MB used)

disk2s4 - VM (APFS) [APFS VM] (2.15 GB used)

disk2s5 - j***y (APFS) (15.23 GB used)

disk2s6 - Update (APFS) (668 KB used)

disk1 - APPLE HDD ST2000DM001 2.00 TB (Mechanical - 7200 RPM)

Internal SATA 6 Gigabit Serial ATA

disk1s1 - EFI (MS-DOS FAT32) [EFI] 210 MB

disk1s2 [APFS Container] 2.00 TB

disk3 [APFS Virtual drive] 2.00 TB (Shared by 6 volumes)

disk3s1 (APFS) [APFS Container] (15.43 GB used)

disk3s1s1 - H********e (APFS) [APFS Snapshot] (15.43 GB used)

disk3s2 - Preboot (APFS) [APFS Preboot] (401 MB used)

disk3s3 - Recovery (APFS) [Recovery] (1.12 GB used)

disk3s4 - VM (APFS) [APFS VM] (18.25 GB used)

disk3s5 - H********** - Data (APFS) [APFS Virtual drive] (1.22 TB used)

disk3s6 - Update (APFS) (4 MB used)

shows your computer shipped with Apple's Fusion Drive system, which consists of a small SSD linked by software to a larger mechanical mech hard drive. Your two drive components have lost that link with each other. If Fusion is working, both sections of the drive description will contain the word "Fusion." It's not present in your report.

Based on the drive speeds in the report, you are running off the much slower mech drive:

Performance:

System Load: 1.55 (1 min ago) 1.56 (5 min ago) 1.69 (15 min ago)

Nominal I/O usage: 1.04 MB/s

File system: 36.56 seconds

Write speed: 102 MB/s ⚠️

Read speed: 136 MB/s ⚠️

For a healthy Fusion drive, those speeds should be Writes between 600 and 900MB/sec and Reads over 1300 MB/sec. You Fusion drive is "split."

—What to do: First evict the junkware from your computer, including App Cleaner. macOS, cat-like, cleans itself and has for nearly a quarter-century. It needs no help licking itself. You also have pieces of Norton. Must go.

As D.I Johnson wisely observes, adding RAM can't fix sick storage. The best of all worlds is to get the Fusion system again working up to its potential.Once the background clutter is evicted, see this Apple Article:

How to fix a split Fusion Drive - Apple Support

Would an external SSD help? Marginally, IMHO, based on your current drive scores, but not like getting the Fusion working. If one of the Fusion components has failed, the external SSD remains an option to get a little more use out of this computer without a significant parts replacement effort.

An SATA 6GB SSD in a USB3 enclosure set as the boot volume will post write/read spesds of about 400MB/sec. Yes better, but let's get that sick Fusion drive to the ER first before going external.

Once the Fusion drive is working we can revisit some other issues that might help.

"CleanMyMac" claims another victim.

D.I. Johnson wrote:

If this were my Mac and I was having these issues, I would seriously consider wiping the drive and starting over with a fresh install of macOS.

👍

In fact that is usually the only effective remedy.

As Allan Jones noted that Mac's startup drive is not performing well. Whether the aforementioned "cleaning" product was a cause or an effect will never be known. Rule 1 of Macs is don't install junk.