[Quoth]

Originally Posted by Hzj_jie

Thank you for your reply, I do know what I am talking about. I need extra weeks to get the statistics for a meaningful comparison.

Impossible without a control device or fresh battery to swap.

Originally Posted by Hzj_jie

But for the questions here, slowing the device or draining battery unnecessarily are both unacceptable, no matter how good the excuse is,

I only ever heard of Apple doing that. There has been no suggestion of Amazon doing it. Apple makes 30% to 70% margin on their products. Amazon wants you to buy the ebooks and/or have a Prime sub or a KU sub. They make little selling their Fire or Kindle models. The profit is on the content.
I've a Paperwhite 3 and I know someone with a Paperwhite 1 and also people with several generations of Basic. I also have other older Kindle models that all needed new batteries (cells). One needs a 2nd battery replacement now. I see battery life suddenly get much poorer, but never due to a FW upgrade. I've two PW 3 models. One is on last FW as are my other non-PW Kindles. No effect on run time from firmware.

Originally Posted by Hzj_jie

at least there should be a config to flip back.
Also, I am talking about using exactly the device but only downgrading the firmware, no other changes.

I agree there should be, but AFAIK there has never ever been the ability to go back on FW on a Kindle like on a Kobo, or updates on Linux and Windows. I've no idea how to "go back" on Android either. It seems vaguely possible on Chromebook, if you did a FW backup first, as it can reload FW from a USB device at boot time.

I've been using Kindles since before the Voyage was released and the most likely thing is your battery (really an LiPoly cell).

The Voyage came out in late 2014 and ceased about Q3 2016, so the battery/cell is about 8+ to 10+ years old. That also is about the lifetime of an LiPoly cell, though reduced by number of complete charge cycles (10x 10% = 1 cycle) and elevated temperature.

The PW3 in mid 2015 was a budget alternative, the 2nd 6″ model with a 300 dpi screen. The Oasis 1 (6″) is really the mythical Voyage 2 in mid 2016 with Voyage discontinued a few months later. The two later Oasis models are 7″.

Occam. No-one else seems to be claiming that a FW update makes a significant difference and last Voyage update was Version 5.13.6 – April 2021 That means almost FOUR years for people to complain.

Firmware Versions for Kindles. From Amazon.

Timeline of Amazon Kindle Models. Probably USA dates. Some models are months later for rest of world, or EMEA.

An LiPoly cell can last for even 15 years or as short as 5. There are a lot of variables including materials quality. The 8 to 10 is most common and they do degrade faster beyond a certain point.