Latency is the time delay for data to travel from a sender to a receiver and back. In networking it is usually the round-trip time (RTT) in milliseconds. For web pages, people also use it loosely for the wait before the first byte arrives, which adds server processing time to network delay.
Why it matters
Latency is bounded by physics and multiplied by the number of round trips. Light in fiber travels at roughly 200,000 km per second, so a 10,000 km path cannot have an RTT below about 100 ms. A new HTTPS connection needs several round trips before data flows, so high latency makes sites feel slow even with plenty of bandwidth.
Example
ping -c 4 example.com
# rtt min/avg/max/mdev = 21.3/22.0/23.1/0.7 msAn RTT of 22 ms is fine. With 150 ms RTT, a TCP handshake plus TLS 1.3 handshake costs about two round trips, around 300 ms, before the request is even sent. Moving content closer with a CDN or reusing connections addresses this directly.
Common mistakes
- Confusing latency with bandwidth. A bigger connection does not reduce round-trip time.
- Testing only from your own city, which hides what distant visitors see.
- Looking at the average only. Tail latency (95th or 99th percentile) is what users complain about.
- Blaming the network when the delay is slow application code or database queries.
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