Speed · Plain-English answer
Yes, slightly. Encryption wraps each packet in extra headers, which adds a small percentage of overhead on top of whatever you download, with WireGuard among the leanest protocols. A VPN does not bypass your data cap, because your provider still counts every encrypted byte that crosses your connection.
Every packet you send through a VPN is wrapped in an encrypted envelope with its own addressing information, and that envelope costs a little space. The percentage varies with packet size and protocol, but for typical browsing and streaming it is a small single digit share of your total, with WireGuard adding less than older protocols.
There is also a trickle of keepalive traffic that maintains the tunnel while idle. It is tiny in absolute terms, though worth knowing about on very small mobile allowances.
A persistent myth says a VPN gives you free or uncounted data. It does not. Your ISP or mobile carrier counts everything that crosses your line, encrypted or not, so a capped plan reaches its cap marginally sooner with a VPN, never later.
What the encryption does change is visibility. The carrier sees how much you used but not which sites or services the data went to, which defeats throttling rules aimed at specific traffic types rather than overall volume.
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