External boot SSD running slow

it is connected to a port where it is running as usb

  External USB Up to 480 Mb/s USB

this mean you will only get a sustainble transfer rate of around 40MB/sec

where your internal hd even if a slow 5400rpm will likely give you double the transfer rate

if it was me then I would connect the ssd to a thunderbolt 2 port

iMac (Retina 4K, 21.5-inch, Late 2015) - Technical Specification - Apple Support

guitar_garage wrote:

It is connected via usb-c to usb-a cable.

I’d wonder if this is simply a USB 2.0 cable.

Find a USB 3.x cable, and try that.

A USB 3.x cable might be labeled SS or SS¹⁰ or 5Gbps, while an older USB 2.0 cable might have no labeling, or nothing past the USB branching logo. USB 2.0 has one row of metal connection pins inside thr metal shroud, and variously a white or black plastic insert, while USB 3.x will have two rows of connector pins and maybe a blue or (if faster) some other color. Older USB labeling was atrociously bad.

https://sct-supply.com/blogs/articles/usb-2-0-vs-usb-3-0-explained-in-one-table-how-to-identify-usb-c-version

What is that Kingston SSD is plugged into..?

It looks like it is being slowed down by a Generic USB 2.1 hub.

Or by some other devices that are plugged into the Generic USB 3.2 hub.

If possible, you should plug the SSD directly into the iMac and plug your other device into the hub(s)

To add to Rudgear's recommendations that mode has 2 Thunderbolt 2 ports capable of 20 Gpbs speed. You should get an Thunderbolt 2 to USB adapter and run the external boot drive from a Thunderbolt 2 port.

Also you've installed what many of the experienced users here consider akin to malware: MacKeeper. You should uninstall it according to the developer's instructions. The use the shareware app Find Any File to search for any files with a particular file name.  For MacKeeper software you'd do the following search(es): 

1 - Name contains mackeeper

Any files that are found can be Option dragged from the search results window to the Desktop to get a copy of the file.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.

The same should be said for the many Ad-bockers you have installed:

Ad-blockers:

        [Loaded] Language - /Applications/Ad Blocker.app

        [Loaded] Security - /Applications/Ad Blocker.app

        [Loaded] Privacy - /Applications/Ad Blocker.app

        [Loaded] Adult - /Applications/Ad Blocker.app

        [Loaded] Ad Blocking - /Applications/Ad Blocker.app

        [Loaded] Custom - /Applications/Ad Blocker.app 

Only one should do. After looking at the above it appears that you have only one AdBlocker installed: Ad Blocker.app. If that's the case ignore the previous about ad-blockers.

For what it's worth I use Ghostery AdBlocker for Privacy. It's unobtrusive and very effective.

1) For some reason that Kingston External SSD is only running at the slower USB 2.0 speed.

Test it on a USB-C to USB-A 3.0 cable plugged directly into one of the iMac's USB-A 3.0 ports.

2) You have a lot of third-party App's and Extensions all running at the same time.

Consider reducing those or replace that iMac with on that has an SSD and more RAM.

3) You have installed multiple third-party Cleaning and Protecting software.

Completely uninstall MacKeeper and BuHoCleaner as per the developers directions.

4) Why are your Gatekeeper and Apple Security disabled....?

Your SSD seems slow. Have you connected it via usb c?

Also make sure that you enable gatekeeper, remove the orphan files

Re: “if it was me then I would connect the ssd to a thunderbolt 2 port”

You cannot connect USB devices directly to Thunderbolt 2 ports.

It would not be worth buying a Thunderbolt 3-to-2 adapter plus a powered Thunderbolt 3 dock with a USB port just to add a USB port. Especially since the USB-A ports on a 2015 Mac support USB 3.0 speed.

"BINGO"

You're welcome.