In the spirit of all around Apple hate, I have jumped the bandwagon on this one :D
Apple has admitted to intentionally slowing down older iPhones, without notifying users. Source
“Last year we released a feature for iPhone 6, iPhone 6s and iPhone SE to smooth out the instantaneous peaks only when needed to prevent the device from unexpectedly shutting down during these conditions,”
“We’ve now extended that feature to iPhone 7 with iOS 11.2, and plan to add support for other products in the future.”
My iPhone 6 speed was nonexistent after iOS 11.2 update, even taking a screenshot took 3 seconds. After checking the CPU speed, I was kinda relieved, it has shown a little above 1100MHz and with lower battery percentage it went down to 800MHz. Battery health was at 86%.
So I ordered the cheapest replacement with adhesive strips in EU part of Ebay and done the transplant following ifixit instructions.
The phone is back to 1400MHz and the speed bump is really noticeable. So if you have half of hour, some simple tools and steady hand, go for it :).