Effiency needs to be worked on for next release of Opera Mini Or version 15, if it is too late to make the needed improvements for version 14.
Version 13 which is the latest version for iOS 7and 9 is NOT running very smooth, with hiccups, generally laggy and sometimes unresponsive, on My iPhone 4 and 4s which have the latest versions of iOS 7 and 9.

I have recently switched from an iphone 3GS with iOS 6, which is the latest ios that 3GS supports. Opera Mini version 6, which is the latest compatible version for iOS 6 is from my experience definitely smoother and for most times without the irritating hiccups and laggyness in version 13 on iPhone 4 and 4S.
The only way I,ve found to make it somewhat smoother to use is to set "Data Savings" to "Opera Mini" instead Of the other option "Off" which somehow makes the problem even worse sometimes.
I Believe it started to go wrong with Opera Mini 8 as it seems to have made major changes from version 7 which some users complained about in the forum.