Megabits and megabytes
Internet plans are sold in megabits per second; files are measured in megabytes. Dividing by eight converts between them, so a 100 Mbps line tops out around 12.5 MB/s.
This single factor of eight is why downloads feel slower than the advertised number suggests. Nothing is wrong with the connection.
Why you never hit the theoretical speed
TCP/IP headers, retransmissions, encryption and the server's own limits all take a share. Wi-Fi adds contention and signal loss on top. Eighty to ninety percent of the line rate is a realistic expectation on a good wired connection; Wi-Fi is often lower.
For large transfers the bottleneck is frequently the source server or your storage device rather than the link itself.
What speed you actually need
Streaming standard definition takes about 3 Mbps, 1080p around 5 Mbps, and 4K between 15 and 25 Mbps. Video calls need 2 to 4 Mbps in each direction. Ordinary browsing and email barely register.
The number that matters for a household is concurrent demand, not headline speed. Four people streaming 4K simultaneously need roughly 100 Mbps; the same connection is idle most of the rest of the day.
Latency is not bandwidth
Bandwidth is how much data can move per second; latency is how long a single packet takes to make the trip. Video calls, gaming and general responsiveness depend far more on latency and jitter than on raw throughput.
This is why upgrading from 100 Mbps to 500 Mbps often changes nothing noticeable for everyday use. Large downloads finish faster; web pages do not load meaningfully sooner, because they were never bandwidth-limited.
Frequently asked questions
Why is my Wi-Fi slower than my plan? +
Wi-Fi is a shared, half-duplex medium subject to interference, distance and competing devices. A wired test is the only way to check what your line actually delivers.
Why is my upload speed so much slower? +
Most consumer connections are asymmetric by design, on the assumption that people download far more than they upload. Fibre connections are often symmetric; cable and DSL rarely are.
Does a faster router increase my speed? +
Only if the router was the bottleneck. It cannot exceed what your line delivers, though a better router can improve Wi-Fi coverage and handle more simultaneous devices.