Why its impossible to find private key with the public key

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Why its impossible to find private key with the public key
Based on this article:
https://medium.com/coinmonks/private-and-public-key-cryptography-explained-simply-4c374d371736
For what I understand in the article, the public key is generated by performing `n' tangent + mirroring operations from G point (having G point defined at secp256k1 and begin n a natural number that directly relates to the private key)
My question is: as I know the public key and the seed value `G', I could start a process computing G·G, G·G·G, and comparing those results with the public key value until it matches.
If your answer would be that this process is very slow I have another question:
If this process is slow, the same applies to, knowing the private key, and compute the public key (Because the operations are the same except for the check operation), so how it is possible to compute the public key in a very fast way?
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