I switched from Sparrow to Airmail [1] a few months ago, and it's pretty much as fast. It has all of Sparrow's features -- Gmail and Dropbox integration, unified inbox, search across folders, multiple sender identities, conversations etc. -- plus some minor ones that Sparrow lacked, such as the ability to undo sending.

You need to follow the beta releases [2], though. The Airmail releases have always struggled with minor bugs. Things like some random spaces being removed from emails, or it displaying the wrong email (!). Nothing to ruin your day, and I'm sure it will stabilize eventually.

[1] https://itunes.apple.com/us/app/airmail/id573171375?mt=12

[2] https://rink.hockeyapp.net/apps/84be85c3331ee1d222fd7f0b59e4...