In the first sentence of the article you linked:
> You can downgrade (If you don’t have Shsh blobs saved) only until Apple is still signing the old software.
Saving SHSH blobs needs to be done in advance, while the iOS version you want to install is still being signed by Apple. With blobs, you used to be able to use a replay attack to force a restore. This doesn't work anymore due to the addition of NONCE, except in very limited and not particularly useful circumstances (you can move to an unsigned iOS version IF you're already Jailbroken AND if SEP is compatible).
If you know a way around this system, please do share. The Jailbreak community will love you for it. :)
Apple does everything in their power to make downgrading iOS impossible outside of a narrow window. I suspect this is also why we haven't seen any actual, definitive speed tests between different iOS versions. The discussion always devolves into "this feels slower" or "I remember my phone used to be like this," because no one can actually install an older version to test with.