its very oftenly missunderstood that "greater the ram of the phone, better is the performance",but  performance of a phone merely depends upon its ram (assuming the phone has lpddr4 ram) the main part of any device is its processor that effects its performance.

ISSUES WITH ONEPLUS 3 6GB RAM:
in early days oneplus used pretty aggressive memory management and it did not allowed user to use ram more than 4 gb of ram , after 4gb ram consumption the processes in the background apps are automatically closed.
  
so, why having ram greater than 4gb can be disaster?
because more ram will hold more number of processes at a time , and the processor will have towork on all the applications currently stored in the ram ,
the processor will have to do lot of unnecessary processing and that will result in extra power consumption.
facebook app alone takes upto 350mb of ram , along with it if several heavy process is running in the background it will easily add upto 3gb tp 3.5gb .

 if ram management is not done properly then it can easily decrease phones battery life.

ram is just a storage that holds the current running process that is currently being processed by the processor.
GREATER THE RAM , MORE NUMBER OF PROCESSES IT CAN HOLD AT A TIME