Problem Calling Function Twice At The Same Time

Hello There,

I am currently making a game using Qt and C++. In the game I have a small function called MoveTo and this function moves an NPC to a specified location. This works perfectly fine. However, If two NPC's try and use the MoveTo function at one time, one will wait for the other to finish the function which can be very annoying when game making.

Is there another way around calling the same function multiple times at the same time?
I have already tried Threading but the performance takes too big of a hit especially on my crappy dev pc.

Thanks in Advanced Guys,
Matt