I just changed my network topology and added a PC behind a openwrt router, downloads from the PC's perspective have slowed down from around 35 MB/s to 25. When it's downloading I see high load on the router, and it mostly seems to be in kernel land.
Under the display options on htop setup, selected the "detailed cpu time" so you can see more granular information about the cpu usage, whether it's some kernel land routine or whether it's interrupt processing