Objects vs. the L2 Cache
An excellent little performance optimization presentation that shows how important memory layout is for today's processors:
The beginning of the talk makes the observation that since C++ was started in 1979 the cost of accessing uncached main memory has ballooned from 1 cycle to 400 cycles.
The bulk of the presentation shows the optimization of a graphics hierarchy library, where switching from a traditional OO design to a structure-of-arrays design makes the code run 3 times faster. (Because of better cache access patterns.)