Starting the snapshot client causes massive amounts of system lag, including keyboard input delays and what I can only describe as FPS drops. This affects the entire system while Minecraft is open, even in the main menu (but not if only the launcher is open).
All 1.9 snapshots are affected. I have not experienced this in any previous versions, including 1.8.8.
Environment
MacBook Air (13-inch, Mid 2012)
OS X Yosemite 10.10.4
java version "1.8.0_40"
Java(TM) SE Runtime Environment (build 1.8.0_40-b27)
Java HotSpot(TM) 64-Bit Server VM (build 25.40-b25, mixed mode)
Dupe of MC-83746