I've been using that boot 'speedup' fix for a very long time now. It worked on Windows XP and still works in Windows 10.
You cannot set a number of cores greater than what is already in your CPU chip. That setting will only show you how many cores you actually have and NO more! I always set it to use the max. number of cores available, but..... I never mess with that memory setting.
Obviously, messing with that setting and getting it wrong, can BRICK your computer.
The entire fix for making a PC boot up quicker was written up in a popular web site several years ago and I still have it on file.
Shorten the Boot Time in XP, Vista & Windows 7 & 8
Go to the start button, choose run, then type msconfig and press Ok.
On the system configuration window, choose the "Boot.INI" tab.
Check “No Gui Boot”, then lower the timeout to a more manageable time.
I choose 3 seconds in stead of 30. (windows won’t accept a lower number than 3)
Next choose advanced options.
This is where you can choose how many processors you have.
Most modern PC's are duo core (2 processors) with some quad core (4 processors)
then choose OK. The Windows default is only 1 core.
Now choose apply and OK, reboot and you should see a marked decrease in boot time, and an Increase in Run-Time efficiency.
Some Self Proclaimed experts will say that the fix does nothing, but if done right it doesn't hurt either, so I do it anyway.
It does seem to speed up the Boot-Up process.
If it can do nothing, then why did the MS programmers include it in very version of Windows since at least Windows XP?
Cheers mates!