More cores buys you only so much unless memory bandwidth and preferably number of memory subsystems is also increased (NUMA sockets). It's often easy to consume

More cores buys you only so much unless memory bandwidth and preferably number of memory subsystems is also increased (NUMA sockets). It's often easy to consume all L3/memory bandwidth with just one core. A single Haswell core can load over 150 GB of data per second, 64 bytes per cycle.