SpigotAPI Method on different thread/async runnable slowing

Question

I'm trying to run a certain "scanning" system. It has to scan millions of blocks and do many different things with each block so when I try and run it normally I get: https://sourceb.in/nFuUfzJ1oj this crash. So I tried putting it in a runnable and running it asynchronously however it started fast, then kept slowing down and slowing more until it pretty much made it so it would be weeks before it finishes. Like that. What's happening here and how do I fix it?

I then tried doing it in a separate thread,

new Thread(new Runnable() {
   @Override
   public void run() {
      // code goes here.
   }
}).start();

but still the same issue