Why does your internet speeds fluctuate or get slower over time?

Answer 1

There are many reasons for differences in internet speed and different technical depths in which this question could be answered. On a high level, your internet speed depends on:

1. the bandwidth used by devices within your network ( connected to your router )
2. the interferences caused by surrounding networks ( if you use wireless, your router's channel might need to be adjusted to minimize this issue )
3. the bandwidth used by your neighbours connected to the same knot ( your cable/DSL line is connected to a hub which in turn is connected to a "stronger" cable towards the provider )
4. your distance from said knot
5. your providers capability to handle fluctuation of requests/data ( depending on your provider, bandwidth might be bought from suppliers, just like certain cell-phone providers buy themselves into existing networks )
6. your provider's QoS protocol. This literally means that your provider might intentionally limit your bandwidth to operate cost-efficient or with higher margin.

Steps to take

1. measure you private network. Check your router's channel and interfering networks ( many routers come with built in evaluation tools for that )
2. measure you ISP's performance (e.g. with speedtest.net) for ping, downstream and upstream. Capture information periodically for eventual claims towards your ISP. It's normal to experience fluctuation in the performance.