Booting Windows on nvme takes 15 minutes, SOS

Hello, I'm new here, so please let me know if I do something wrong!
I desperately need help to fix this issue with my desktop, if anybody can help me I'll be forever grateful.
My OS (Windows 10 pro 1803 on Samsung evo 970 M2) used to boot in a matter of seconds (with win fast startup activated), except sometimes rarely it would take much much longer, for unclear reasons. Because this used to happen rarely, I didn't bother to investigate into it.
Recently I have added a new Samsung 870 evo sata SSD to the previous build, and installed on it a second Windows 10 pro, in dual boot. Since then, my old OS has begun to start all the times in that same sort of "slow boot version", whatever it is, only that now it always does that, so it obviously has become a big problem. (my guess is that in dual boot it's not doing the fast startup anymore, and that the regular boot might be hindered by some problem that was already there before, but I'm not a tech person so I could definitely be wrong)

Behaviour: After powering it, and going through the normal sequence of motherboard logo and windows loader selection, if I pick the freshly installed Windows (with no drivers or updates installed yet), it will boot in seconds. If I pick the old one, it will bring back the motherboard logo and the dots circling, and this will go on for exactly 15 minutes, after which it suddenly logs in and starts up in seconds, without any further issues that I can tell. (I would say that I didn't see the light on the case blinking during the 15 minutes process, as if it wasn't in use, but I can't be sure)
What I've tried: turned off "Fast startup" on both the OSs. Disabled hibernation. Installed Samsung nvme drivers. "sfc /verifyonly" reports no violation.

Before attempting to flash the bios and doing all sorts of updates and stuff, I wanted to understand what's really going on during these cursed 15 minutes, so I came to know about the WPR.
I installed the Windows Performance Toolkit to make a boot record, but I realized I don't know which parameters I should activate in Resource Analisys in this situation. (I don't want to risk breaking the computer with a huge recording of 15 minutes of boot, since I don't know exactly what I'm doing. xD )

Can anybody give me a suggestion on how to proceed? Is it possible to reduce the 15 minutes boot to maybe a couple of minutes, to make it more tolerable? Thanks to anybody who wants to help!

Some additional info:
-Because people will always ask why I want two copies of the same system and why I don't do updates, I will just explain it here and be done with it:
the old OS is for my job, which is why I prefer to not update the system and the drivers unless it's really necessary. It happened to colleagues that updates broke the drivers of drawing tablets or other stuff that I need, and I don't want to go through that. For the same reason, I don't want to reinstall the system and lose all my settings and programs. (I also took a system image with the Windows tool before, as I didn't have other softwares to do it, I hope it's good enough in case of emergency.) I have installed the second Windows 10 OS with the intention of keeping the new one updated, to play games on it, although I never got around doing it yet.
-The old OS was activated (NOT BY ME) with a KMS activator by the person that gave this machine to me. I have bought a licence myself and activated it, but (much to my dismay) I was told not to remove the activator, because doing so could break my Windows; so I kept it and added it to the Defender exceptions. One of my concerns is that this "software" could be causing some security issues; could that be related to the long boot? I'm curious to know if there is a way to get rid of it safely.
-I noticed the UEFI is weirdly missing "NVME configuration" menu where the mobo manual says it should be, or at least I couldn't find it; but the nvme disk appears in 'boot priority', both as disk and as Windows Boot Manager.
-the nvme is behind the graphic card, so I haven't tried to "unplug and replug" it because it's impossible to reach it without disassembling everything...
-Did I write too much? ^^; Please, forgive me.

Desktop machine specs:
Motherboard: GA-AB350-GAMING3
CPU: Ryzen 5 2600x
GPU: RTX 2070
RAM: 32 GB Corsair Vengeance DDR4
Samsung evo 970 M2 (Windows 10 Pro 1803)
Samsung evo 870 SATA (Windows 10 Pro fresh install)
HDD for storage

Dalchina, as I said, Windows IS legally activated on this system! Do I really have to make a clean install just to remove this useless activator that wasn't even installed by me? Would you guys be willing to give me assistance if I managed to remove it? Do you have any suggestion to where I could ask assistence to its removal without having to wipe my whole system? I don't want people to work for free, I would pay a technician for assistance, if I knew someone reliable who would actually help instead of telling me to reinstall the system.
Thank you... :''<

Jo2503, you have a second hand computer that was activated with a crack (most of them has malware), you have a system that should boot in seconds and takes minutes and you're hesitating to do a clean install, deleting all Windows partitions.

A Clean Install Windows 10 should have been your first act.

I understand that a clean install would probably be easier, but then it would take me a lot of effort to bring back all my softwares and customisation. (And if it's a bios problem instead of Windows, it might not fix things either, am I correct?)
After the sad 15 minutes of just idling nonsensically, the system just wakes up normally, boots in seconds and runs smoothly as always, and hasn't given me troubles so far, maybe I'm naive but I doubt that it is infected by anything serious other than that damned kms. Therefore I wanted to make at least attempt to understand if there is something that can be done. I thought analyzing a trace of this slow boot could at least give be a vague idea of what might be faulty, since that's the only instance when the system faces the issue... Could you at least point me to the correct settings to perform a meaningful recording of the boot with the WPR?
If nothing relevant comes up, I suppose I'll have to try to flash the bios and make a clean windows install, eventually.

First step is to update the BIOS.

Thank you everybody for bearing with me. ;^;
I'm not discarding any suggestion, even removing the dual boot or the clean install. I just wanted to get a picture of the situation because I don't know where to start.
I did a recording of the boot with WPR, and it recorded only about 135 seconds (which is maybe not a great startup time for a nvme, but not so bad either, right? I'd be ok with that). I'm not sure what happens before that, but at this point if my poor understanding is correct, what happens before it might be caused by the BIOS, so like Megahertz said... maybe that's where I should start? It did feel suspicious that the NVMe configuration menu did not appear anywhere...
Is it best to flash it from bios or to update it from the Windows gigabyte utility? Looks like the process to update from Qflash could be quite complex, from what I read in the forums. I have f23, I should update chipsed drivers, install f31, install EC FW Update Tool, install f41... not too newbie friendly, but I've also read that to fix problems flashing bios is better, is that true? x_x
Thank you again for your patience.