Adding multi-threading?

[Wouze]

Hey I'm kinda new to c++ stuff and I've been trying to use multi-threading, because I just have watched a video about it :)..

I want to ask you, why this approach doesn't work?
it seems to have no effect on the CPU usage.

this is the original code for one of the most time-consuming tasks:

for (Colony& colony : colonies)
{
    colony.update(dt, world);
}

and this is what I done:

for_each(std::execution::par,colonies.begin(), colonies.end(),
        [&dt, this](Colony& colony){
             colony.update(dt, world);
         });

May I have your opinion?