Call vector destructor or clear in a different thread

I have huge vector of unique_ptr .

When I need to do clear() or call destructor it took 4-5 seconds.

One theoretical way to speed it up is to create temp vector, swap it with the huge vector I work, then start a thread and call clear on temp vector.

When looking over internet i do not see anyone speaking for optimization like this.

Is there a flaw In my optimization?