Network tx/rx processing likely. Possible causes
- SQM or any other QOS
- Lots of iptables rules
- You're using a not-very-powerful router with an old 500mhz 32 bit mips74kc cpu dating from 2011
You can try to enable software flow offloading on the main firewall tab.
If that doesn't help, without meaning to be facetious, get a newer and more powerful router