Most C++ implementations of std::string will allocate a new block when appending a string (unless the implementation is specifically tuned to always allocate extra space after a string assignment); and then the data from the old block is copied to the new block, and the old block is freed. The memory pool implementation doesn't have to allocate a new block. It just extends the current block and so it gains the speed advantage.

It would be a fairer benchmark if the String class were templated and it took an Allocator where the allocator could either be the standard C++ allocator, or the MemoryPool allocator. That way it can be shown that the same operations are being performed, but MemoryPool is actually faster.