Latency is the time it takes for a packet to travel from one point on the network to another and back — usually measured in milliseconds.
Explanation
Latency (also called round-trip time or ping) is dominated by the speed of light in fibre plus intermediate router hops. Within a metro area, latency is 1-5 ms. Cross-continent (New York to London), typically 70-90 ms. Halfway around the world (US to India), 250-350 ms. Latency dominates the perceived speed of interactive applications more than raw bandwidth does — a 1 Gbps link with 300 ms latency feels slower than a 10 Mbps link with 20 ms latency for browsing and gaming. Web proxies add one extra hop (client → proxy → destination) which adds one leg of latency; picking the nearest proxy region minimises the impact.
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