Actually, I spent a good 7 years in the video games industry, working everything from high level game logic to graphics programming to performance optimisation. On that last point, OOP is a complete dog when it comes to performance. If you have an update loop with a bunch of heterogeneous objects, you'll be increasing your instruction and data cache misses, by having to load vtables and method definitions. I haven't seen a situation in a game update loop where composition couldn't provide better flexibility as well as performance benefits.
What games did you work on (or kinds of games, if that's too personally identifiable), if I may ask? I'd be curious what sort of games have so many objects flying around that the overhead of vtables and cache misses was a significant factor in your overall performance.
I worked in the industry myself for a year or so, on an MMO (back when they were the Next Big Thing) and while our game wasn't super optimized, OOP overhead wasn't even on our radar as an issue compared with rendering, wrangling art assets, Scaleform UI elements, dynamic loading of the same, and all the other bits and bobs.
(Not arguing that composition isn't often better, just that performance isn't usually a significant reason why.)
