FYI - I didn't notice a difference between dev 5.x and 6.4.1 on two devices with relatively poky processors (but not memory constrained). Search results were promptly presented and navigation between message and results was nearly instantaneous. On both devices I tested before and after installing the 6.4.1 update, from the search bar and using swipes, from inbox and 'all mail', and with and handful and several screens of results. I do not use the smart folder. Hopefully did not miss the targeted use case.
There is no 'complaint' here as execution/navigation was fast in all cases! Just sharing outcomes given this is a dev build subject to testing.