Slow download speed

My download speed keeps intermittently dropping below 2mbps with minimal home usage for the last few weeks. I’m paying for up to 100mbps, and while I recognize it will generally be slower, when I check speed through xfinity it’s been 4-25mbps and and when I checked through another website it’s been <1-14mbps with only 2 devices using the internet. The intermittent slowdowns happen most frequently at night & speeds are fine most of the time so I assume this is a network issue, not an issue with my hardware.

I checked that I’m well under the monthly data limit. Does the network have an issue with video streaming? Anything else I should check? Or a way to keep download speeds to the house at at least 5mbps?

Is there any real difference between starter and performance internet when performance is so much slower than advertised?