Here's some scientific analysis:

Re-do the speedtest in DC during off-peak hours (11pm, 3am, 5am) and see if you have increased speed. If you do in most often of these cases it's a case of congestion plain and simple if the speed tests were in the same location. Try doing a set outdoors.

It's already known in the tech media that VZW is ALREADY experiencing congestion <2Mbps in HIGH population density areas of the US (LA, SF, DC, NYC, etc.) The solution for this is increasing cell site density (building more cell sites) or adding capacity using AWS which VZW has already activated in parts of NYC.

Sometimes the only solution in these cases is to wait until VZW adds more capacity. You can get a more accurate confirmation if the CSR's I believe send it to their network team which will tell you a definitive answer (and yes it's congestion and it's already occurred in Manhattan for example).