In articles I've read and personal testing, performance increase is from
15%-50% or more depending on the quality of the SSD drive. The Win10 Insider Testers group which I'm part of have reported increases in this range.(there are currently over 5 Million testers in the group). My understanding that earlier versions of windows such as 7/8x don't give such a dramatic increase, as the OS itself is not as efficient as the W10 kernel. Lastly, our recommended drives tested show top brands to consider are Kingston, Intel, and Samsung. I can tell you that when I put in my Kingston 120GB SSD on my 8 year old Sony Vaio laptop, it was like it was a brand new computer again on Win7. When I upgraded to W10, it was even better than new!!
Earlier versions of Windows from say Win98-Vista only show 15%-20% max performance increase in testing I've seen across major SSD or SSHD drive brands.
Hope this information proves helpful.
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