Google may provide an OS Patch in the future allowing you to turn off encryption which should solve it but we don't know for sure. it seems like it would be an easy fix and give the choice to the customer. not everyone needs encryption and the performance hit!
Agreed, 100%. If there were some glaring security issue that left Android devices exposed and at substantial, realistic risk without this new encryption, I think we'd all be fine with it and expect it as an update to all devices even without necessarily getting Lollipop. Or, if added levels of security can be added without substantially impacting performance, this would probably not even be a topic of much note.
But if the impact is as substantial as recorded in AnandTech's study, and this is like mandating installation of a speed governor to a vehicle to keep people safer by limiting it to 45mph, it should be an optional item. While it's true most will not notice it, not everyone wants or needs it, and those should be given the option to disable it.
For my purposes, the most read/write-intensive task I expect I might do would be 4k video recording and transfer. From what I've been able to scour, 4k 30fps video recording on mobile devices runs outputs in the vicinitiy of a 48Mbps bit rate. I'd be interested to learn the difference in time it would take to transfer say a 10 minute video at that spec with or without encryption - with a file that large, it could be a considerable difference.