Why is the ART VM slower than the Dalvik VM, at sorting an array when the array size is 62000?
Most probably the test is inconsistent. Have you tried repeating the test at least 100 times to have pseudo-valid statistic results? And are you sure the emulator is a valid platform on where to test such low-level things? It may be optimised for the Dalvik VM.

Best guess is to try it on a real device, like the new Nexus 5 or waiting for the Nexus 4 and Nexus 7 updates.