Some Internet providers cache or compress online data to save bandwidth (especially cellphone networks). It is a good idea to test the issue by connecting to an entirely different network, preferably from a different service provider.
They can be performed with a minimal amount of technical knowledge.
You'll want to work with a tech support specialist to ensure that your AV software is not interfering with Steam.
It may also be necessary to add exceptions for Steam in your AV configuration (for example, excluding Steam Library folders from live monitoring services).
Any software that modifies or monitors system, disk or network activity can cause issues while downloading or running a game.
These programs and services can run in the background and are typically associated with software like Anti-Virus, ad-blockers, web-optimizers, firewalls, system cleaners, recording software, proxies, performance boosters etc.
You'll want to work with a tech support specialist to ensure that these services & programs are not interfering with Steam.