Clubins iperf3 server -- iperf.clubins.de:5201/tcp

Upload (you -> server):
  iperf3 -c iperf.clubins.de

Download (server -> you):
  iperf3 -c iperf.clubins.de -R

Parallel streams, if one stream cannot saturate the link:
  iperf3 -c iperf.clubins.de -P 4 -t 20

One test at a time. "the server is busy running a test" means exactly
that; wait and retry. UDP (-u) is not open.

Browser speed test:  https://speed.clubins.de
Raw download files:  https://files.clubins.de  (also plain http://, no redirect)