I never recall any version of iOS ever being rumoured to be a bug fix optimisation realise.
There was a small rumour that iOS 8 that:
"In addition to the in-app tweaks, Apple has been working to speed up iOS 8 in comparison to iOS 7. Applications are said to launch more quickly, close more quickly, and overall system navigation is said to feel much smoother and more stable. Apple sped up animations and improved stability in the recent release of iOS 7.1, but the enhancements in iOS 8 are said to go a bit further.
This seems to relate to the animations and slightly to system performance, and while this did no turn out true, this was only a small part of the rumours for iOS 8. Most focused on other things, such as health kit, siri with Shazam etc. Optimisation in iOS 9 has been consistently rumoured, with very few other things suggested.
iOS 7 had no rumours on Macrumours about optimisation. Sme with iOS 6.