Use the PACKET_IGNORE_OUTGOING socket option for dhclient
On Linux, dhclient opens a packet socket to facilitate low-level operations. This means that it has access to all the packets that are sent or received on a given network interface. A BPF filter is used to efficiently ignore irrelevant packets, however there is still some kernel overhead involved in filtering those packets. In the majority of use cases, the overhead is negligible, but under very extreme circumstances that overhead can be as much as 6%.
Linux 4.20 and newer supports a new socket option (PACKET_IGNORE_OUTGOING) that allows outgoing packets to be ignored for a packet socket with even greater efficiency. As best as I can tell, dhclient is only interested in incoming packets, so it may be possible to add this feature without any large changes or loss of functionality (I hope).
I believe something similar exists for BSD distos as well.
I am willing to test and verify unreleased engineering code for this feature and contribute whatever feedback I can.