I'd guess that many gameObjects could be the problem, when you probably don't need them to be. Could make node be a regular class (no inherit from MonoBehavior) and create it the normal programming way:

LinkedList<Node> N; 
N = new LinkedList<Node>(7500);
for(int i=0;i<7500;i++) {
  N[i] = new Node();
  // math to set node values from whatever
}

If you actually have 7500 gameObjects, that people have hand-entered data into, you can still suck that into the Nodes at run-time and delete them.

Also, a List is good for fast anywhere inserts&deletes and OK for going from front to back. It's pretty awful for looking in spot i (for example, if Node 106 links to 390 and 5602.) For random access like that, an array is much faster.