No.
Vector uses two values: size and capacity. The size is the actual number of elements stored in the vector, while the capacity reffers to the allocated reserved space in memory. The performance increase comes from allocating more space than is needed.
Only when the request for resize which will result in a capacity greater than the current one is performed, will the re-allocation take place. Also, if you try to insert a lot of elements in the middle of the vector, it will impact on the algorithm performance since all elements after the inserted ones need to be moved (sometimes, this can trigger reallocation, if you exceed capacity of the vector).
You can use the reserve member function to further increase speed: the reserve member function will make sure that the capacity is set to a specific value.
You can read more about std::vector on page 148 - in the book The C++ Standard Library: A Tutorial and Reference.