Running Win 10 64-bit, I had converted from an SATA SSD to NVME after I'd built the initial system between Nov 2016 and Feb 2017. I'm using a Samsung "Pro" NVME as the boot disk, and never noticed any problem like this.
I built a twin to that system. The original has 32 GB of RAM, the twin has 64GB. On boot-up, it takes longer before one hears the "beep" followed by system post.
I really don't sweat boot delays in the matter of "how many seconds", though. I either sleep or hibernate the machines. Restart or Shutdown/Cold-boot may give me time to sip some coffee or my morning juice.
As for Dell or any other OEM, I haven't purchased an OEM box since 1994, except for my laptop. Which -- I almost hate. Instead of an LG Gram, I wish I'd got an Acer Nitro 5. But it's not an important issue or problem. I'd rather work at my desk with the desktop PC, and I use the laptop when I go out on the patio to sit under my canopy.
JUST AN AFTERTHOUGHT: even if the OP lets the thread die or nobody comes back to it.
At boot time, BIOS goes through a drive/storage enumeration process, then posts the drive information for devices connected to the motherboard storage controller. If, for instance, there is a non-bootable CD in the optical drive (if you have an ODD these days), it will stall. I could wonder, with an empty ODD whether the sequence set in the BIOS could delay the enumeration.
I'm just speculating here. I mentioned that it takes longer if there is considerably more RAM in the slots. I would also investigate the storage situation -- how many drives connected, which ones take longer to be counted at startup, etc. etc.
But I wouldn't hesitate in any event to make the system boot from NVME with any chipset as old as mine or newer. As for converting from SSD (1st Z170 system) as opposed to original OS install on NVME (2nd Z170 system), I don't see any difference or problem.