iPerf involves no writing to the disk, all test data is only ever in memory. Those devices performing better just happen to have better CPU performance to be able to better shuffle the packets around.
However, there's a few public iPerf servers from around the world, listed on the iPerf website: https://iperf.fr/iperf-servers.php
One thing to keep in mind is that the client is sending by default on iPerf. If you want, you can use the -R option to have the server send instead, to test against these public iPerf servers, in the downstream direction.