It is reasonable to use a whole HDD just for OS, but if you have like 1TB of HDD and your just gonna use it for OS, then that is unreasonable. If you feel your HDD is too big just for OS, you can partition it to different sizes, 1 Physical HDD, 2 Drives shown in My Computer.
The relative amount of free space available in your Drive that has the OS is important, having too low of available disk space does slow down the computer (reaction time/reading time). But the "time and speed" of the HDD is limited to its specifications, especially cause it's an HDD, other than that, you can try a single SSD maybe 60Gb/120Gb/240Gb just for OS and some main programs that you need. It will definitely by 100% increase your loading times speed.