Xcode 7 unit testing extremely slow

... cannot merge previous GCDA file: corrupt arc tag (0x00000000)
... cannot merge previous GCDA file: corrupt arc tag (0x00000000)
... cannot merge previous GCDA file: corrupt arc tag (0x00000000)
... cannot merge previous GCDA file: corrupt arc tag (0x00000000)
... cannot merge previous GCDA file: corrupt arc tag (0x00000000)
... cannot merge previous GCDA file: corrupt arc tag (0x00000000)
... cannot merge previous GCDA file: corrupt arc tag (0x00000000)
...

Xcode 7 is extremely slow when unit testing, ASAIK there's two ways to boost it up.

1. Use iOS 8 simulator
2. Turn off Generate Legacy Test Coverage Files and Instrument Program Flow in your projects's build settings

GCC_GENERATE_TEST_COVERAGE_FILES = NO;  
GCC_INSTRUMENT_PROGRAM_FLOW_ARCS = NO;  

Still I got some strange behaviours on testing, and I don't know why and how to fix it.
When unit testing, my object not been released(dealloc) immediately, the timing seems a little bit deferred.