Solved: Memory Upgrade - HP Support Community - 8355935
Timetec 16GB KIT(4x4GB) Compatible for Apple DDR3L 1600MHz PC3L-12800 CL11 for iMac (Mid 2011, Late 2012, Late 2013, Late 2014 Retina 5K, Mid 2015 Retina 5K) SODIMM Memory Module MAC RAM Upgrade at Amazon.com
TIL: You can view CPU frequency and temperature in htop : r/linux