Researchers from the University of Massachusetts Amherst and Northeastern University ran 650,000 tests within the U.S. from 2018 to 2019 to see if wireless carriers were slowing down (throttling) streaming services. The full report can be found here: https://wehe.meddle.mobi/papers/wehe.pdf

They concluded that regardless of when the tests were conducted, the throttling happened consistently. Carriers will often respond by saying user activity peaks and affects network speeds.  ATT throttled Netflix 70% of the time and YouTube 74% of the time, but didn’t slow down Amazon Prime video. T-Mobile throttled Prime video 51% of the time, but didn’t bother with Skype and only occasionally slowed down Vimeo. Google, Amazon, and Verizon contributed to the report.