Good day and thank you Justin!
As I said, I haven't actually timed it, it's a perception.
It could be latency, as you pointed out. I'm in New England, AWS-East is somewhere in Northern Virginia (I believe), whereas DO is in California. As I reckon it on the mapsof.net distance calculator, it's slightly less than 500 miles to VA and about 2,700 miles to CA.
Assuming that Internet traffic to both places would travel on a direct line from my computer, the 2,200 could be very significant to latency.
I'm actually surprised that the results of your tests were that close. Uploading a single 25 MiB file may not be enough to measure the difference in speed.
If I have time this weekend, I'll test the transfer speeds with a few hundred MiB of assorted files (or perhaps I'll just keep my computer off and enjoy the weekend).