Very interesting. I would have bet on one or memory leaks. I used iOS 8 on a 5S until just last week. iOS 8 on my 6+ is just way buggier.
I'm thinking it is just a software glitch on the 6 and 6 Plus. I say this because look at the 5s and 5 on iOS 7.0.x. The 5 ran OK, meanwhile the 5s had bug after bug. Could just be the OS isn't fully optimized for the iPhone. This has also gotten me thinking that maybe Apple was planning on putting 2GB of RAM in, thus decreasing the need for a heavy compression engine for RAM. This would create extra battery life since it wouldn't have to constantly be compressing and decompressing.
Maybe there wasn't enough time to re-optimise the OS because another last minute decision was made to decrease the memory. Or, its entirely possible Apple's dev units had 1.5 or 2GB of RAM and the final production had 1GB. This has happened the past; a shining example would be the original Mac back in 1984. Most of the dev units inside Apple had 512KB of RAM while a last minute decision (either by the board or Steve I forget) bumped it down to 128KB.
It's software related all the way.
Just look at how well 7 ran. There isn't much more going on to justify the performance drop.
I expect it will be the same situation as when iOS 7 came along. That wasn't all smooth and gracious when it landed, think people forget that.
An 8.2 (or whatever it is) will make the difference.
I hope so. I would expect something like this to be really fully fixed until the early part of next year but Apple could surprise us. I really think 8.1 should have been 8.0.3 because all it really did was enable Apple Pay. 7.1 launched with a heck of a lot of bug improvements including a performance increase for iPhone 4. 7.1 launched after 7.0.6 in March 2014.
Point being here, iOS 7.0 got 5 improvements (5 because one was only for Chinese iPhones) before iOS 7.1 came out. iOS 8 got literally
one. Yeah, sure it says 8.0.2 but really 8.0.2 is 8.0.1 repackaged to fix the issues with 8.0.1. If 8.0.1 hadn't taken the "phone" out of so many iPhone 6's, it would have gone: 8.0, 8.0.1, 8.1.
IMHO, Apple should have done iOS 7.5 with more bug improvements, optimization for iPhone 6/6 Plus, and possibly some of Continuity. Leave the new APIs, programing language and other underlying tune-ups until they were ready in iOS 8. Maybe hold off on 8.0 until March, or even June or Sept.
Maybe next time they will do a Public Beta of iOS. It seemed to be popular with OS X Yosemite and it seems to have paid off; Yosemite is probably Apple's most stable release of OS X since Snow Leopard.