What Is the Computational Problem That Makes This Derivation Hard?
The computational problem is the Elliptic Curve Discrete Logarithm Problem (ECDLP). Given two points on the curve, P and G, where P = d × G, the problem is to find the integer scalar d (the private key).
For the secp256k1 curve used in Bitcoin, the size of the number d is so large that there is no known efficient algorithm to solve for it in a reasonable amount of time, even with massive parallel computing.