I can open LR 6.4 successfully but after a few mouse clicks I get the spinning beach ball. LR becomes unresponsive for about 2 minutes and then seems to operate normally. I have made no system changes and have tried with and without GL and also tried in safe mode with no luck.
Lightroom version: 6.4 [ 1060496 ]
Operating system: Mac OS 10
Version: 10.10 
Application architecture: x64
Logical processor count: 8
Processor speed: 2.0 GHz
Built-in memory: 16,384.0 MB
Real memory available to Lightroom: 16,384.0 MB
Real memory used by Lightroom: 1,556.0 MB (9.4%)
Virtual memory used by Lightroom: 2,671.3 MB
Memory cache size: 799.8 MB
Maximum thread count used by Camera Raw: 8
Camera Raw SIMD optimization: SSE2,AVX,AVX2
Displays: 1) 2880x1800, 2) 2560x1440, 3) 1920x1200
Graphics Processor Info:
Intel Iris Pro OpenGL Engine
Check OpenGL support: Passed
Vendor: Intel Inc.
Version: 4.1 INTEL-10.6.33
Renderer: Intel Iris Pro OpenGL Engine
Got your Lightroom preferences and turn off GPU acceleration. This often helps resolve this situation.
Thanks! Tried that with no effect.