What is the incentive for a bitcoin node to broadcast the newly received block as fast as possible?

But why would a node help other nodes

Because anyone with any stake in Bitcoin needs the existence of a healthy Bitcoin network in order to be able to use their money and for it to retain any value.

Since the network is a mesh, there are, in general, no strategic nodes that can maliciously act as a bottleneck or filter. I would expect miners to take care that their selected peers are diverse.

Even if most nodes were malicious, it is very likely that almost any honest node would be in contact with at least one other honest node and would detect the malicious nature of the malicious nodes.

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