MTU
MTU is the largest packet that can be sent into a network in one piece. On ordinary wired networks that is 1500 bytes. A tunnel adds its own headers to every packet, so less room is left for data inside — say 1420. If an application sends a packet of the old size, it will not fit and has to be split. Splitting is provided for in principle, but many networks discard the control messages used to negotiate it as unnecessary. The result is a distinctive picture: short requests get through, while the first large response goes nowhere — and the connection does not fail with an error, it simply hangs.
In short
The symptom is recognisable: sites open, but a large page, image or file stalls completely, and only on some networks. The fix is lowering MTU in the connection settings; 1420 or 1380 is usually enough. Protocols over TCP rarely hit this because segment size is negotiated when the connection opens, whereas for tunnel protocols like WireGuard it is a classic.
Frequently asked
How do I know it is MTU?
By how selective it is. With an MTU problem small requests pass normally while large responses hang with no error message, and it happens on one specific network rather than everywhere. If nothing works at all, or everything is slow but working, the cause is something else.
What value should I set?
Start at 1420 and drop to 1380 if the symptom persists. The point is for the packet plus tunnel headers to fit within what the path allows. Going too low is not free either: the smaller the packet, the larger the share of overhead and the lower the useful throughput.