The instance of the launcher that is running in the background creates a memory leak. I'm not sure if this is snapshot related or not but it has always dragged my game down overall in the live game. Simply killing the process usually solves most of my performance issues.
When using the latest snapshot however after about 2 minutes of spawning and killing entities the ticks per second in minecraft completely crash down to even below 1 tps. This renders the game unplayable as there are no redstone or other updates happening. This is crucial when developing custom maps that use redstone and command blocks (duh).
Having to tell your users that play your map to kill a process is a bit hacky and advanced so I prefer to finally get this issue resolved. It has been haunting me for over a year now.
After having the lag (but before killing the launcher): Please attach the complete output of the "Development console" which can be found on the second tab of the launcher.
If the launcher closes after game start, please edit your profile and select "Launcher visibility" then, "Keep the launcher open".