My iMac is extremely slow running

Orphan files are files that the app that used them is no longer on the Mac. They can be deleted to save storage space.

Part of the problem is that the model you purchased was the slowest that Apple offered at the time of purchase: 8 GB RAM and 5400 rpm hard drive.

Other, and maybe more serious, are the installation of Webroot and MacKeeper. That are considered to be akin to malware by many here.

I recommend uninstalling them according to the developer's instructions. Then you can check to see if you've removed all of the supporting files by downloading and running the shareware app Find Any File to search for any files with the application's or the developer's name in the file name.  For the two apps in question you'd do the following search(es): 

1 - Name contains webroot

2 - Name contains mackeeper

3 - Name contains macpaw

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

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

If you get warnings that the file can't be deleted because it is in use or used by another app boot into Safe Mode according to How to use safe mode on your Mac and delete from there.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

You should also reboot more frequently. I recommend shutting down at night. This will clear out temporary system and application swap and cache files which, if damaged, can cause performance problems.

That's the 8 gigabyte hard disk drive iMac. That was slow when it was built.

Why is it slow? Why is my hard disk drive iMac so slow? - Apple Community

What can you do, short of replacing the iMac? Use an external SSD as your startup disk … - Apple Community

You can also use the AirPrint drivers, as the specs indicate HP ENVY 5660 e-All-in-One Printer series supports AirPrint. That means no add-on drivers are needed for most operations. That means less stuff installed. That also gets rid of the need for that unsigned printer-named app (from HP?).

There's also some hunks of an add-on Webroot anti-malware app, and a cleaner app. Some anti-malware and some cleaners can cause performance problems, and some of the better-known add-on security vendors have themselves been privacy problems. What happens with some of these add-ons that are or become incompatible or that have been incompletely removed can be anybody's guess. And if you're having problems with limited performance, you want most everything not completely necessary removed. Here, use the built-in anti-malware, and the built-in cleaning.

What to do, pending future replacement with a better-configured Mac?

• Remove Webroot, and remove MacKeeper.
• See if AirPrint printing to that printer works.
• Migrate to and boot from an external SSD.

+1 to OT's recommendations. After you uninstall (and double check it's gone!!) the software OT's recommends getting rid of you may want to consider getting an External SSD to help make the computer quicker. However considering the computers age, you may want to consider getting a new Mac mini which would be a HUGE upgrade over your current computer. If that is something you would consider you will need an external monitor (they start at <$100) and the iMac. You could use the old iMac's keyboard and mouse with the new MM. For about $1100 you would have a brand new Mac that will far outperform your current machine, a 1 year warranty and of course it is eligible for AppleCare, all things your old Mac don't have.

To migrate from the old machine is dead easy so if you are interested in a new Mac let us know and we can provide instructions on migrating from the old machine to the new one.

By the way, Yukon Territory is fabulous. We took up trip up there a few years ago, lots of incredible scenery and wildlife!