I had to switch from Airmail to Apple Mail in the latest DP6 because Airmail is just working slow now. It could be an issue that they need to fix in their app but Apple Mail seems to be working great now, faster than before I switched to Airmail back on El Capitan. It feels like less memory usage when left open also...
Same here for me. Airmail is taking forever to download/display mail. Had to switch back to Apple Mail for the interim.