Have you tried removing the HDD and booting to see if the boot times come back to normal? When you start the system at the Dell splash screen start tapping F12. Choose Windows boot manager and see if the boot times are normal. Or if the SSD is listed try that as well as I suspect that some of the windows files are on the HDD. Also at F12 choose diagnostics and run full system diagnostics as there could be a problem with the HDD or SSD. Pay attention to the drive results.
I will try this later tonight, but I did try something similar prior. I went to bios, system configurations, drives, and disabled the sata ports. Upon booting I was greeted by what I think is the Intel Rapid Storage software saying "No Hard Drive", then immediately shutting off the machine before windows loads. Boot partition is definitely on the SSD.
After removing the hard drive it booted like it should. I see u suspect windows files on the HDD but unless there is some black magic involved this is physically impossible. The computer was reset and windows was reinstalled with the hard drive out of the system.