Every speed test shows two big numbers. They measure traffic in opposite directions, and they matter for very different things. Knowing which one is limiting you saves a lot of guessing.
Watching a video, loading a page, installing an app or a game update all download data. Download speed decides how sharp your video can be, how many screens can stream at once, and how long big files take.
Your camera and microphone on a call, photos syncing to the cloud, sending a large file, live streaming a game, and home security cameras sending video out all use upload. When upload is too low, the people on your call see a frozen or blurry picture even when you see them clearly.
Cable and DSL connections divide their capacity unevenly, giving most of it to download because that is what most homes use. A plan sold as 300 Mbps may come with only 10 to 35 Mbps of upload. Fiber plans are more often symmetrical. Check the upload figure your plan actually includes before assuming something is wrong.
These are planning figures with some headroom. Add them up for everyone who uses the connection at the same time, or let the bandwidth calculator do it.
Upload time is size in megabits divided by speed. A 10 GB backup is about 80,000 megabits. At 10 Mbps that takes about 2.2 hours; at 50 Mbps, about 27 minutes. The Mbps converter helps with the units.
Even downloads need a small upload stream for acknowledgements. If a backup fills your upload completely, those replies queue up and browsing, calls and games all slow down. If your speed tests look worse while a backup runs, limit the backup's speed in its settings or schedule it overnight.