  Dell XPS 9500 installed a 1TB Samsung 980 PRO PCIe 4.0 as a secondary SSD drive Could it be something to do with the fact that it's a PCIe Gen 4 SSD? Not likely as all the parts are designed to fall-back to the supported PCIe version. That is standard "backwards compatibility". I would look else-where. Does machine still boot properly with the new SSD removed? Was the new SSD completely blank before install? If not, DiskPart clean it now and start setup-steps over.  
I've recently installed a 1TB Samsung 980 PRO PCIe 4.0 as a secondary SSD drive to my Dell XPS 9500. Since then the PC takes forever to boot and stays at the "Dell" loading screen for ages. I've already changed the storage mode from RAID to AHCI, but it has not made any difference. Have gone through other boot options such as Fast boot and secure boot to see if it made a difference, but to no avail. Could it be something to do with the fact that it's a PCIe Gen 4 SSD? Any help would be much appreciated.   Kind regards
Dell XPS 9500 Extremely slow boot after 2nd SSD installed