I'm not aware of speed test sites that allow you to select the location (i.e doing a speed test from cloud provider A to Hetzner).
I'll look into what's going on by running some CLI speed tests on hetzner.
iftop provides traceroute / MTR information. which may show where the 'slow down' is (may not be because of Hetzner or the receiving cloud provider).
wget -O /dev/null 100mbsized.test.file
iperf & iperf3 allows for active measurements of the maxpossibile bandwidth with support for tuning various parameters related to timing, buffers and protocols. this would allow for changing/tuning sysctl.conf internet settings as appropriate. iperf/iperf 3 server on herzer and client on other cloud provider.