Company network is getting advertised Mbps download speed, yet downloads are abysmally slow.

I recently became an IT support tech at a startup that only has myself and one person above me in our IT department. My boss was does not have a networking background so most of the networking problems are giving to me even though I would not consider myself a network expert by any stretch.

If you test our network speed using a speedtest like fast.com we are getting our advertised speed, ~100Mbps, but we only get 1MB/s actual real world download speeds, if we're lucky. Is this some sort of configuration issue that is eating our download speeds? Shouldn't we be getting 9-10MB/s?

I know this question is remedial, but due to the nature of "Accurate speed test speeds, slow download speeds." I have found it difficult to find an answer for this on Google and forum posts. Thank you for any help or advice on the issue.