Computer is running excessively slow. Can't find solution

My Windows 10 is running excessively slow.
A few symptoms:

When I right click my desktop, explorer freeze for 10-15 seconds while the context menu loads.

Games will attempt to load but will take 5+ minutes to get into the program. Most times games will freeze while attempting to load.

Non-game programs will not start initially and will take 15-30 seconds to load.

Booting into windows takes much longer than normal. legacy speed was 15-25 seconds. Recent has been a few minutes.

Computer specs.
i7-4790K processor
Gigabyte Z97-D3H Motherboard
Radeon R9 390 Nitro 8GB - Graphics card
16GB Hyper X (4x4GB sticks) - RAM
Sandisk SDD 250GB - OS drive
Seagate 3TB drive 7200 RPM - Main HDD

Things I have tried.
Cleaning up my SDD to below 55% free space on drive
Ran sfc /scannow. No errors.
Ran DISM repair. No avail.
Shut down most startup items using ccleaner and msconfig. - very little affect.
attempted No GUI Boot
Turned off Windows Search - Didn't work
- Disabled Cortana
- Reenabled Cortana - Both had no affect on problems
Flashed BIOS with most recent update - No affect

Running out of ideas, and I am looking at just reinstalling my OS. The computer has problems with everything except for internet searching it seems.

Any assistance is greatly appreciated.

Thank you

you possibly have a virus

Run Malwarebytes free, if it doesn't download, you may have to do it in safe mode...here's the link, and let us know what it finds https://www.malwarebytes.org/dl-confirm/ As Simon says, it may be a virus

I had done that as well, unfortunately forgot to add that. It did find something, but didn't fix the issue. Trying in safe mode now.

Ran in Safe Mode. Nothing found. No affect to fix performance issues.

I found the culprit. The hard drive is failing. Did a read/write test yesterday and it was making out at 2.5 Mbps with a lot of reallocated sectors.

If you can't clone it, checkout SpinRite software for fixing your HD (grc.com). Ancient software that can even help SSD. You will have to pay for it, but it can repair problems that no other software can. If SpinRite does the job, you will then be able to clone the disk (presuming you are not able to clone it now).

AnEdge said:

Run Malwarebytes free, if it doesn't download, you may have to do it in safe mode...here's the link, and let us know what it finds https://www.malwarebytes.org/dl-confirm/ As Simon says, it may be a virus

Saw this while trying to sort EXCESSIVELY SLOW performance from W10x64 Home. I had run Malwarebytes Premium full scan and it found 2 instances of BrowserHelper.DLL and labelled them as PUP probably unwanted programmes. After asking Malwarebytes to delete these I see there may be some utility in having them (or, one of them). But they are gone.

My PC is back to its normal speed.

Good to see the ref to Malwarebytes; it can help in some circumstances.

As good as having the PC back to a healthy state.