Threaded Pathfinding still lagging

Well, we have threaded pathfinding in our project, it’s A* with a node grid, I thought I can outsource the calculations on other cpu cores with threading, but it’s still affecting the main core. I took a look at the Windows TaskManager to see the CPU heat, and all 4 Cores are affected by the calculation. Is there any possibility to keep the Thread-Job on just one CPU core?
We tried to only start one thread-job at a time, but this doesn’t change anything, all 4 cores are on their limit.

thanks in advance

Edit: it’s not just the pathfining, we tried it with different calculations, like filling a list of floats and a testClass in a for loop

What are you using for threading? ThreadPool should evenly distribute jobs to other cores/CPUs.
How big is the graph you’re pathfinding around?

It may be worth splitting into smaller chunks and finding a path to the closest point in that chunk to your destination, and repeat when you get to the edge of that chunk (or whenever else, so long as the job is split).