It is great if you can reproduce this! We got similar complaints a few times but the users who reported could never do a thorough investigation.
Maybe it is due to girder not implementing some less frequently used HTTP GET features correctly. For example, girder does not implement file chunk download correctly (therefore resuming an interrupted download results in a corrupted file).
It could be interesting to see if download speed depends on time of day, depends on what software you use, and it could be interesting to have a look at the HTTP GET request to see if there is anything different from the browser in that jetstream2 instance compared to the browser on your computer.