Does my iMac 27 late 2015 have pci-e (apple nvme) slot

since the iMac was available with a Fusion Drive, there would be a space for a PCIe drive even if it was not originally configured with the Fusion Drive option. but if there's still doubt, you can go to OWC’s website and contact their support to confirm that. they really know their stuff when it comes to Mac hardware. and you could kill two birds with one stone because I would recommend OWC to get the SSD from.

but I will echo Jack19's question. why not get an external SSD and run the Mac from that? but if you feel confident in your skills to self-service the iMac, then yes an internal PCIe drive would definitely be faster than an external SSD.

yes I forgot about that with the 2015 iMacs. that is 100% correct. the Mac must have been configured with an SSD and / or Fusion Drive to have that slot available on the 2015 models. my bad. :(

Hello!

As iMac was never designed to be repaired or upgraded by the end-user, and these modifications have great risk of damage or permanent failure to the iMac, it is highly recommended you use an external SSD for macOS. Why do you have hesitations?

Jack

OWC model info states that a 21.5-inch iMac must originally have had a Fusion Drive or factory SSD to upgrade to a blade SSD, That suggests to me that the slot is not present in yours.

Also chenc OWC footnotes. There seemt o be a lot of "except fot 2015" note for their expsnion pasrt. See:

https://eshop.macsales.com/shop/ssd/owc/imac-21.5-inch/2013-2019

and this footnote:

Also RAM is soldered in that model. Not even the minor deities on Mount Cupertino can add RAM to that model once is leaves the assembly line.