All the best, highest performance AAA games have been written in C++.

So have all the slow, under-performing, crash prone, unoptimized AAA flops.

C++ isn't a magic bullet that gives instant speed. Indeed a long running native app that isn't properly written, will fragment the heck out of memory and get slower and slower. (With 64bit address spaces, at least they don't crash now days!)

The choice of algorithms and data structures is of utmost importance. Optimizing around one's run time is choice #2. C++ doesn't provide some magic hyper optimized environment. Heck C++ doesn't provide much of anything. You need to build a threading model and concurrency model that works, and performs well on different platforms. File I/O performance is different based on the file system in use, NTFS for example hates lots of small files, other file systems have less issues with that usage model.

This goes on and on and on. With C++, every decision has to be made, and each one of those decisions can, if made incorrectly, lead to a poorly performing app.