Maybe you can use a heat exchanger, like in nuclear power plants, and separate the data center flow from the outside flow, so they can go at different speeds.

You would use a heat exchanger normally anyway. Forcing the outside (DH) to be slow would get you that, but there is cost in having low flow in that HXs are less efficient at the far end and you can transfer less heat in the same pipework (it would more than half the district heating capacity). So in practice, not really.