You might need to make a modlet and not modify the game files.
There are tutorials on how to make modlets on the forum: https://community.7daystodie.com/topic/7653-xpath-modding-explanation-thread/
You can use the following for the traders.xml in your modlet folder:
<configs>
<!--...
In windows task manager go to the details tab, find the program you want to set and right click then click Set affinity.
I wonder if this is a Unity3d issue.
It looks like Subnautica had a similar problem. This mod sets the CPU affinity to 3 cores and they claimed it doubled the FPS...