Why is it faster to specify the length of a vector object?

The fixed parameter of the vector constructor is not redundant. A vector object with a fixed length is faster than a vector with an unfixed length. This is something I knew long ago. But I really don't know why. Today, I have no intention of finding the answer in the Adobe Document: each time the vector size increases, a new memory block will be allocated. The current content of the vector is copied to the new memory block. This additional distribution and replication of data will reduce performance. Therefore, try to set the length to a fixed value when creating a vector object.