@Mm1900
In router mode, the load will not be distributed between Main Deco and Satellite Deco. It is because all network traffic goes through Main Deco.
In addition, Main Deco must manage all Satellite nodes and connections to them, which adds to Main Deco load.
TP-Link recommends to limit number of Satellite Deco wirelessly connected to Main Deco: How many Deco units can I add to my Deco mesh network?
You can check CPU and RAM utilization on Main Deco. Use its IP address in browser, which should be http://192.168.68.1 if you haven't changed defaults, and login to Main Deco Web interface. Scroll down to the bottom of the page and you will see:

That info is updated in almost real time, no need to refresh web page. Give it few moments to settle down after initial spike and you will see actual Main Deco utilization.