51% attack clarification
Yes, the attacker needs to be faster than the rest of the network combined.  Being faster than the second-fastest miner is not enough.

I think this is where you are mistaken:


  Blocks in the mainchain can only be added sequentially, hence the growth speed of the blockchain is at most the speed of the fastest miner in the network


Yes, blocks are added sequentially, but I can't follow your logic as to why that means block growth only happens at the rate of the fastest miner.  If you are the fastest miner on the network, that in no way means that you are producing 100% of the blocks.  It means that you are producing the highest proportion of the total, which is still likely a minority.  Block growth still happens at a rate of the entire network collectively, regardless of who is fastest.

With that in mind, you can see why an attacker needs to produce blocks faster than the rest of the network combined.