旅行中使用 VPN：打开之前该知道什么

Not necessarily. Permanent use costs speed, battery and diagnostic clarity when something goes wrong. A more balanced pattern is on for unknown Wi-Fi, off for ordinary use over your own data plan. If your employer requires it to stay on as a matter of policy, follow that and budget a little extra data for the overhead.

Slightly, yes, because wrapping the traffic adds headers to every packet. How much depends on the protocol and on the apps you use, so no fixed figure applies. What is certain is that there is no separate channel: it all counts against the same allowance. If you sized the plan tightly and plan to run the tunnel continuously, leave some margin.

Turn the VPN off for a moment and try again. If it works, that app is refusing VPN connections, which is the bank's own anti-fraud behaviour rather than a fault on your phone. If your VPN supports per-app exclusions, set one up so you are not toggling every time — and test this before you travel rather than at the counter.

We cannot answer that for you: the rules differ by country and change over time. Check the destination's own official sources before travelling, and if the trip is for work, check with whoever handles this in your organisation as well. We do not advise using one to work around any rule or restriction.

No, and it usually makes it marginally worse, because the data travels further and carries extra overhead. If the connection is slow because the signal is weak or the cell is busy, the constraint sits at the network layer, where a VPN has no influence. In that situation the useful moves are to turn it off, close unused apps and do one thing at a time.

The question is not the price but who you are moving your trust to. Once the tunnel is up, that provider occupies the position the network owner had, so choose on the clarity of their policy and their track record rather than on cost alone. Swapping a network you know nothing about for a service you know nothing about does not make you safer.

It does not affect network attachment, but a misconfiguration can look exactly like a dead connection, so turn it off as an early troubleshooting step.

No: it adds a small overhead. To save data, lower video quality and disable background sync instead.