In your launcher parameters make sure the following things are set:
Platform: 64bit
CPU Count: 8
Extra Thjreads: All on
Enable Hyper-Threading: On
Memory Allocator: Intel TBB4 (this is the biggest one!)
Enable large-page support: On
System memory limit: Set to 14336 (14gb so your OS can keep the remaining 2)
Report back if that helped.