For that kind of network task on this kind of hardware it is not normal to have so much CPU time used up for networking alone. But since this also about kernel-task:
Seeing that WiFi is much less affected then cable connections on this machine:
If you look at the temperatures your machine generates then you might find it pretty high. kernel_task is probably not so much about networking anything here but in this case trying to help prevent any damage that might occur through overheating.
In this case kernel_task just does nothing meaningful except blocking too much computation on the CPU to lower the temps. See here for a fuller explanation and of course the official stance on this. It's ultimately down to a rather optimistic thermal design that falls a bit flat if challenged.
Provided you have already checked that your fans and fins are clean and not blocked with cuft: That means your best bet is to try lowering the overall temperature of the affected machine. Raising the unit, at least operate it on a hard, flat surface, may be blow on it with an external fan, cranking up an AC might all help here. Then there are the software solutions to raise the fans rpms. smcFanControl or the commercial ones: TGPro, MacsFanControl, to name a few.