For good performance of NG, it is critical to have a system disk with good random read performance. In final version, there will be no hard SSD condition, but rather a 16kB random read performance requirement (2 MB/s for example). Also it will be possible to install NG through installer even if disk performance condition isn't met.
It is possible to check the performance of system volume by "winsat disk -ran -read -drive c" command.
C:\>winsat disk -ran -read -drive c
Windows System Assessment Tool
> Running: Feature Enumeration ''
> Run Time 00:00:00.00
> Running: Storage Assessment '-ran -read -drive c'
> Run Time 00:00:09.52
> Disk Random 16.0 Read 1.77 MB/s 4.1
> Total Run Time 00:00:10.62
Well there goes my WD5000AZRX Caviar Green 
Gordon.
I should be fast enough then ?C:\Windows\system32>winsat disk -ran -read -drive c
Windows System Assessment Tool
> Running: Feature Enumeration ''
> Run Time 00:00:00.00
> Running: Storage Assessment '-ran -read -drive c'
> Run Time 00:00:05.94
> Disk Random 16.0 Read 2.90 MB/s 4.6
> Total Run Time 00:00:06.33