Polymorphism with objects implies an indirection to data with variable size, and also usually an object graph involving pointer chasing; lots of indirection is bad for performance. Also, cache-efficient manipulation favours non-indirected, tightly-packed data.

So instead of:

  class Entity { Vector location; Foo foo; Vector velocity; }; // etc.
  Entities[] entities;

One prefers:

  Vector[] locations;
  Vector[] velocities;
  Foo[] foos;

A physics engine updating locations and velocities won't waste cache on redundant information (like foo) in this case.

There was an excellent presentation (from some games conference) on this topic posted here on HN a few years ago, but I can't find it easily.

Informative reply.