The main issue with 64-bit is the increased memory footprint, especially in pointer heavy code. However most instructions still deal with 32-bit quantities, and the new ISA and extra registers ensure you do more work using fewer instructions, so 64-bit does usually give a speedup without increasing power. We seen it before with x64 and it equally applies to ARM64.
However do you have evidence of a huge drop in battery life in iPhone 5???
Actually, 64bit architecture is slower than 32bit one with higher power dissipation. The major advantage of 64bit architecture break 4Gb memory access barrier. People wait to see any usage breakthrough for 64bit mobile architecture now.
Qualcomm, NVidia and MediaTek are set to release several 64-bit designs in Q3/Q4 this year (A53 for QC, A53/A57 for MT and Denver). So it's closer to 1 year after Apple. Samsung hasn't announced parts yet, but said it will move to 64-bit SoCs this year. And then there is Seattle, X-Gene and several other server oriented chips, all for this year. It's going to be interesting!
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