Why your VPN is slow, and what actually fixes it

A slow VPN is almost always one of four things: the server is far away, the server is busy, the network you are on has forced the app onto a slower transport, or the connection underneath the VPN is slow to begin with. Encryption itself is rarely the problem on a modern phone. The fix is different for each cause, so the first job is working out which one you have.

First, check the connection without the VPN

Turn the VPN off and try the same thing. If a page or download is just as slow, the VPN is not the cause; weak Wi-Fi, a congested cell tower or a slow site is. A VPN adds its own cost on top of the network and cannot subtract anything from it. If the VPN-off test is clearly better, carry on.

The server is far away

Every packet takes a detour through the VPN server before reaching its destination. Distance costs roughly 1 ms per 100 km each way, plus whatever the route adds, so a server on another continent can add well over 100 ms of round-trip time. That does not always cut your download speed much, but it makes every page feel slower to start and makes calls and games laggy.

The fix is a closer city, but "closer" means closer on the network, not on the map. The route from your carrier to a nearby city can be worse than the route to one further away, which is why the nearest server is not always the quickest. Look at the latency figure the app measures from your phone rather than guessing from geography.

The server is busy

A VPN server has a fixed uplink and a fixed amount of processor time. When many sessions share it, each gets less, and the effect is worse than distance because it changes minute to minute. A busy server shows up as speed that varies wildly, pages that sometimes load instantly and sometimes hang, and a latency figure that jumps around.

This is the cause that switching cities fixes most reliably. Pick a city with more headroom even if its latency is slightly higher. Server load explained covers why a lightly loaded server 500 km away often beats a full one next door.

The network forced a slower transport

VPN apps prefer WireGuard over UDP because it is light and quick. Some Wi-Fi networks, typically in hotels, campuses and offices, drop UDP or filter VPN protocols. A well-built app then tries other methods until one works, and the one that gets through on a restrictive network is often a TLS-wrapped transport on TCP port 443.

That transport is slower by design. It runs TCP inside TCP, so when a packet is lost both layers back off and the connection stalls harder than it should. It also has to use smaller packets, with an MTU of 1280 bytes, so there is more overhead per byte carried. If the VPN is slow on one particular Wi-Fi network and fine on mobile data, this is almost certainly why. The post on networks that drop UDP explains what the app does about it.

The fix, when you have the choice, is a different network. On a restrictive network the slower transport is the price of having any tunnel at all.

Things on the phone itself

A few phone-side causes are worth ruling out:

• Battery saver. Aggressive power modes throttle the processor and the radio, and some slow every background service including the VPN.
• A second VPN or a filtering app. Two apps trying to own the tunnel fight, and the loser drags the winner down.
• Private DNS. Android's Private DNS setting can add a round trip to every lookup on top of the VPN's own resolver. Setting it to automatic usually helps.
• An overloaded phone. Rare, but a phone under memory pressure drops packets it should be forwarding.

Which fix for which symptom

Culvert VPN measures a live round trip from your phone to each city and shows real headroom for each, then connects you to the healthiest server in the city you tap, so the two most common causes are visible before you connect; it is free on Google Play.