Why is my file transfer slower than expected?

A network connection rarely transfers files at its advertised bit rate. A 1 Gbps connection has a theoretical ceiling of 125 MB/s, but the speed you see during a real copy can be lower for several independent reasons.

1. Protocol overhead

Your file is not the only data crossing the network. Ethernet, IP, TCP and application protocols add headers and control traffic. Retransmissions can add more traffic when packets are lost.

2. Storage can become the bottleneck

A fast network does not help if the source cannot read data quickly enough or the destination cannot write it quickly enough. Hard drives, SSDs, NAS devices and cloud storage can all impose their own limits.

3. Latency and packet loss matter

Long-distance connections have higher round-trip times. TCP performance can be affected by latency, congestion, packet loss and the amount of data that can be in flight before acknowledgements arrive.

4. Wi-Fi is variable

Wi-Fi link rates are not the same as sustained application throughput. Distance, walls, interference, channel width, other clients and radio conditions can change performance from minute to minute.

5. The server may be the limit

Cloud services, web servers, VPNs and remote storage systems may rate-limit transfers or share capacity among many users. Encryption and compression can also make CPU performance relevant.

How should I estimate transfer time?

Start with the theoretical line-rate result as a baseline, then allow headroom for the environment you are using. For an important backup or migration window, measuring actual throughput on the real path is more reliable than assuming the advertised link speed will be sustained.