In Oracle Database 11g or Oracle Database 12c, Buffer Exterminate wait event is due to using Oracle’s automatic memory management when the MMON process shrinks the data buffer cache to re-allocate the RAM for another System Global Area (SGA) region.

AMM resize operations can affect overall database performance especially the OLTP environment.
If you find AMM resize operation occur, you can look into V$SGA_RESIZE_OPS and V$MEMORY_RESIZE_OPS and find out how many times it is occurring and affecting the performance of your database.

If you find more events of these during peak times of database, you can turn off Automatic memory management (AMM) and can adjust manually the corresponding SGA and PGA sizes.
You can use below SQL Statement to find out various memory resizing operations took place:-

SQL>select START_TIME, component, oper_type, oper_mode, status, initial_size/1024/1024 “INITIAL”, target_size/1024/1024 “TARGET”, FINAL_SIZE/1024/1024 “FINAL”, END_TIME from v$sga_resize_ops order by start_time, component;