The gap between HDDs and SSDs aren't as huge when comparing higher end HDDs to lower end SSDs.
7 seconds is a great boot time, regardless of HDD vs SSD.
If you've recently installed your OS, or have minimal startup programs etc, it'll always boot faster - especially with the right settings enabled in the BIOS.
I boot to Windows 10 from a Samsung 950 Pro PCIe M.2 drive in <10 seconds, and that's pretty respectable.
If you're getting 7 seconds from a mechanical drive - way to go! You might want to share your secret (if you know how you managed to achieve that kind of boot time), so others can maybe benefit.