In Mac OS X 10.9 "Mavericks" a new core system technology called App Nap has been introduced. The feature allows applications to be put to sleep while they are not in focus. The mine craft dev launcher appears to support this functionality, but the actual minecraft.jar that opens does not, and this is where it really matters. When minecraft goes into the background, the system should automatically tell the application to concede resources to the foremost application, but minecraft fails to do this. This feature is necessary to allow the system to remain at top performance even when a graphically intensive application like minecraft is running. A description of app nap can be found at http://www.apple.com/osx/preview/advanced-technologies.html and further information on implementing the technology can be found here: https://developer.apple.com/library/prerelease/mac/releasenotes/MacOSX/WhatsNewInOSX/Articles/MacOSX10_9.html#//apple_ref/doc/uid/TP40013207-CH100 . A document which outlines all of the new core technologies of Mac OS X 10.9 in depth can be found here: http://images.apple.com/osx/preview/docs/OSX_Mavericks_Core_Technology_Overview.pdf I am willing to test any changes made to the code as I am part of the Apple Developer Mac program and have access to all of the latest betas. I have also attached a screenshot of the activity monitor illustrating the problem.