Why does a router heavily throttle internet speed when a VPN client is enabled?

THE ROUTER'S WEAK CPU CANNOT HANDLE THE CRYPTOGRAPHIC LOAD OF THE TUNNEL.

Unlike computers and smartphones, budget home routers are equipped with low-power CPUs lacking hardware cryptography support. Constant encryption of a large incoming stream loads the CPU to 100%, limiting throughput.

Hardware Limitations of Network Processors

Encrypting and decrypting each incoming packet requires intensive mathematical computations. If the router's chipset lacks AES-NI hardware acceleration instructions, queue processing falls back to general CPU cores, causing overheating, increased latency, and severe speed drops.

Ways to Increase Router Speed

Switch to modern protocols with lower CPU load, such as VLESS without extra encryption.
Set up selective routing, sending only blocked resources through the tunnel.
Enable hardware packet acceleration (Hardware NAT) in router settings.
Move the VPN client from the router to powerful end-user devices.