" Swift’s compiler optimizes string usage so that actual copying takes place only when absolutely necessary ",And when is necessary?
This may help you. From here
Although strings in Swift have value semantics, strings use a copy-on-write strategy to store their data in a buffer. This buffer can then be shared by different copies of a string. A string’s data is only copied lazily, upon mutation, when more than one string instance is using the same buffer. Therefore, the first in any sequence of mutating operations may cost O(n) time and space.
When a string’s contiguous storage fills up, a new buffer must be allocated and data must be moved to the new storage. String buffers use an exponential growth strategy that makes appending to a string a constant time operation when averaged over many append operations.