Really impossible to say if you are actually being actively being throttled or it's just the whole network getting busy and slow. I would guess the latter but I don't know exactly how much AT&T throttles you when it kicks in.
With Straight Talk you could be on any of the 4 major carriers depending who's SIM they gave you. In your case you got lucky with an AT&T SIM instead of Sprint. Since you're on a prepaid MVNO you will get throttled during high-traffic times if traffic is high enough to cause congestion. This is because postpaid customers get priority over prepaid on the data network. And prepaid customers of the network you are on, AT&T, get priority over prepaid customers of a MVNO operator like Straight Talk. So, you're kinda down the data network food chain a ways. Depending on the load the AT&T network has to handle vs capacity at high traffic times, there may not be many scraps left.
Then there's the problem of the inconsistency of Ookla's (and almost any other) online speed test. Speed tests are not necessarily testing your connection speed, they're testing the speed of the entire connection between you and the server you connected to. Testing at different times may result in connecting to different servers. The connection could be bounced around the country, the globe and through as much as 20-30 hops (routing/booster servers along the way). Run traceroute on anything, say, the Chicago Times and be surprised.
$ traceroute chicagotimes.com
IMHO the best you can hope to get from running your tests is a very rough guideline.
