The official installer doesn’t use mirrors, and the mirrors that are available are not maintained by The Qt Company. They throttle the downloads from their own server for anyone that isn’t a paying customer.
This can be verified in the installer and also here at the mirrors. Qt Downloads
Examples:
United States Open Computing Facility, University of California, Berkeley
United Kingdom University of Kent UK Mirror Service
Japan JAIST Japan Advanced Institute of Science and Technology
Japan Yamagata University
…
The Qt Company is doing the absolute bare minimum that they have to under the Kde agreement. See this Why do we give so much power to the Qt Company when they don’t actually own Qt? - Help - Discuss (kde.org)