In large contests, generating an archive of every contestant's submissions takes a long time. Because the "export submissions" functionality does this synchronously, it causes a "504 gateway time-out" error before the archive is finished and ready for download.

One way to fix this would be to generate the arachive asynchronously, with a similar UX to generating user outs - you click once to generate the archive, and after a while, when the archive is done, you refresh the page and get a link to download the archive. In this case, care must be taken to keep track of when the archive was generated, as submissions created after that moment won't be included, and the user should be warned about this.

Another option would be to use a streaming response, so that the archive is being sent to the user as it's being generated. This could make the download slower than necessary, but at least the start of the response would arrive sooner, hopefully avoiding timeouts.

Yet another possibility is that this operation is slow for no good reason and could be made faster.