Where is best place to load the Monterey OS on late 2015 iMAC--on 28GB SSD for faster response????

Your iMac uses a Fusion Drive. The small 28GB SSD is used as a cache to make the system faster so you can use an inexpensive but slow hard drive which supports larger sizes. When a Fusion Drive is created, macOS makes the two separate physical drives appear as one virtual drive.

Your slow system performance is most likely due to a failing hard drive. If you can still boot macOS, then check the health of the hard drive by running DriveDx (free trial period). Post the complete text report here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar. You may as well post a report for the SSD as well, but the hard drive is more likely the one to be failing.

You can try running the Apple Diagnostics to see if any hardware issues are detected, but the diagnostics rarely detect problem even when they do exist.

If the internal drives are healthy, then these are the most likely remaining issues:

• Corrupt file system -- run First Aid on the Fusion Drive item.
• Issue with third party installed software....try booting into Safe Mode. If that works, then post an EtreCheck report so we can examine it for clues.
• macOS was inadvertently installed onto the slow hard drive so the Fusion Drive is not being used
• There is some other hardware issue. Disconnect all external devices in case one of them is causing a problem, otherwise it may be an internal failure (Memory, Power Supply, Logic Board)

FYI, macOS Monterey requires about 50GB of storage, so it is not possible to install it to the small 28GB SSD. Even older versions of macOS probably would not fit on it, definitely not enough to do anything useful even if you could.

If the internal hard drive is bad (even if it is not), you can install macOS to an external USB3 SSD which would give you about the same performance as the original Fusion Drive setup when it was new. While some hard drive failures can affect system performance even when booted to external media, you may not need to physically remove the drive. It is possible to configure macOS to ignore the internal hard drive so macOS never tries to mount it.

Not sure what you are asking. What do you mean by "best place to load"?

If you have an SSD, you install macOS on the SSD. There is no "best place". Its just installing it on the hard drive.

28GB sounds like RAM not SSD storage. There have not been such small SSDs in a long time, unless you are referring to the SSD portion of a Fusion Drive. In that case, you can't use the SSD alone. The Fusion Drive works as a single unit and you install macOS on the entire drive. Though the HDD side of the fusion drive is very slow.

What exactly are you trying to determine?

To determine the cause of slowness posting an Etrecheck report may be a better option.

click here ➜ Using EtreCheck - Apple Community

If you are referring to a Fusion Drive, your best option may be to just get an actual SSD and install macOS on that, and remove the Fusion Drive entirely, though its not an easy task, as you would need to take apart the iMac's screen to get to the hard drive.

Perhaps an external SSD over USB may be a better option.

Chasgribbs wrote:

First, the ~24GB SSD in the late 2015 Mac (assume it's a part of "fusion" drive) has nothing on it. Nada. Empty. Maybe that's normal, but it's a head-scratcher. What's it doing for me?

When a Fusion Drive setup is broken, then neither drive will show data or will only partial data.

At the moment the 28GB SSD is doing nothing for you. Assuming the internal Hard Drive is not failing, then you could recreate the Fusion Drive & reinstall macOS onto the Fusion Drive again. Otherwise you can use that internal SSD for some extra storage after you erase it, but if you store any important data on it, then make sure it gets backed up along with your main drive.

Second, the transmission ports on the Mac are a coupla USB receptors and two T'Bolt #2 receptors. Which is better for an external boot drive--now that I've learned that T'Bolt 2 is a Neanderthal and can't be upgraded.

@Phil0124 is correct. With a USB3 SSD you can get about 500MB/s data transfers depending on the SSD (best to get one with UASP support for the most efficient USB transfers, but many external drives do not list that item).

Technically Thunderbolt 2 is faster and better than USB. but you'll be hard pressed to find an actual Thunderbolt 2 hard drive any more. You may be able to use a Thunderbolt 3 or 4 hard drive with an adapter though.