Try using a bootable USB stick with the Win 10 installer on it, rather than a DVD. And disconnect the old HDD and only have the new one connected to blue SATA port the motherboard. Use the Windows Media Creation Tool to prepare the USB stick. Use an 8- or 16 GB stick. Plug that USB into PC with power fully off. Then power on and start tapping F12. When the F12 menu opens, select the option to boot from USB. The installer should launch and just follow the prompts...
My XPS 8900 started exhibiting slow Windows 10 boot and disk access in general. It takes 10 - 20 minutes for Windows to come up, but once it is up, seems to work fine except disk access is slow. BIOS boots Windows without delay. All drivers up to date. I replaced the hard drive with an unformatted one and attempted to install Win10 from a DVD. The install hung indicating to me that this is a HW problem. I disconnected the DVD drive, moved the SATA cable to another port, replaced the cable, reseated the DRAM and other connectors, and after reinstalling the old HD, still had the problem. I ran the DELL full diagnostic twice and it failed with "Full Test error: We're sorry, an error occurred and we could not complete the scan. Please try again." after complaining that the (HD) tests were taking unusually long time to complete. The short diagnostic test succeeds. What could it be?  
XPS 8900 Slow Boot