Definition
Throughput is how much data actually moves through a network connection per unit of time, typically reported in megabits per second. It differs from bandwidth, which is the theoretical capacity of the connection. Real-world throughput is always lower because of protocol overhead, packet loss, retransmissions, and any number of small inefficiencies. A 100 Mbps internet line might deliver 90 Mbps of useful throughput on a good day and 60 on a bad one.
VPN Impact on Throughput
A VPN tunnel always reduces throughput by some amount. Protocol overhead adds packet headers (40-100 bytes per packet, more if obfuscation is involved); encryption and decryption consume CPU; routing through an intermediate server adds physical distance and potentially congested links. As a rough rule of thumb (not a guarantee), modern VPN protocols (WireGuard, IKEv2) on capable hardware lose around 5-15 percent of throughput. Older protocols (OpenVPN, especially over TCP) can lose 30-50 percent. Heavily loaded servers, distant servers, and mobile devices with weaker CPUs all compound the loss.
What Affects It Most
Server distance is usually the biggest factor — connecting through a server far from both you and your destination adds round-trip time that shrinks effective TCP throughput. Server load is the second factor: a popular VPN server at peak hours can drop sharply. Protocol choice is third: WireGuard is consistently faster than OpenVPN. Local CPU matters when running on phones, tablets, or older laptops. For highest throughput, pick a nearby, lightly-loaded server and use WireGuard or IKEv2.