The Maximum Transmission Unit (MTU) is the physical weight limit of a packet. Ethernet is 1500 bytes. But VPN headers add weight.
1. The Math
If you try to send a full 1500-byte packet through a WireGuard tunnel (which adds ~60 bytes overhead), the total size becomes 1560 bytes. The router drops it.
2. MSS Clamping
The fix is MSS Clamping. This tells the TCP handshake: "I can only handle 1420 bytes payload." Without this, HTTPS handshakes often freeze.