Question Windows 8.1 extremely slowPOST/boot time

Hello.
I have Win8.1 on Asus MOBO with 4790K, Samsung SSd and GTX 970 10 years old PC.
Problem. Extremely slow boot time or restart time, up to 10+ minutes. Last one I was watching BSOD for about 5 minutes, but Win booted through).
A year or a bit more ago I had terrible crash and I remember seen error message something like this (not exactly) "either computer or windows can not find correct path for starting windows". I don't remember what I did to fix it, but it was somehow fixed. The result of this fix was increased boot time, but it was not bad, so I lived with that. a few months back something happened and my boot time shortened, it was almost normal, not fast as brand new, but much better than before.
No, after my GPU crapped out (it was 1080) I put my reserve 970, and now boot time increased to extremely slow.
I suspect that I need to fix either boot path, check for corrupt files or something along those lines.
Once Windows loaded, no issues with performance or anything else.
I havean original Windows 8.1 Ho,e full DVD, but no recoverable media created during past years.

On the side note. I am planning to upgrade to Win10 (while back, during free upgrade time I installed Win10, but did not like it and reverted back to Win8.1, and I think that Microsoft remembers my info, so I can do it again, but only after I can fix existing problem). What is holding me back, is my BIOS don't have USB as bootable option listed and WIn10 requires Dual Layer DVDR, which I don't have either. So I need to fix my existing problem first.

Any suggestions/guidance appreciated.
 
Update
Just finish running CMD Prompt with those commands in that order.
DISM.exe /Online /Cleanup-image /Restorehealth
sfc /scannow
chkdsk /f C:

No errors reported, first 2 commands went quick, third one took some time.
Shutting down part of restart got significantly shorter, but still about just under a minute (before was more than 5 minutes), starting up got faster as well (before was close to 10 min), but still a few minutes.

What else can be done?