I don't think there's necessarily anything wrong with asking a question about a Raspberry Pi any more than any other server (whatever its size).

Rather, judging whether the question is on or off topic should be based on what it's actually asking: could this problem apply to a more traditional server, and could this apply to a professional context?

I can't say this particular question was great, but if you re-read and skip the second sentence (the only one that mentions the Raspberry Pi), it certainly becomes a more generic Nginx configuration question:

I've written a basic file server using Django.

When I try to download files from it (multiple large files at once, ~500 Mb each), I can literally download only one or two at once. [...]

Removing the Raspberry Pi reference is indeed one of your suggestions, and this would probably work in this case. Do you then care on which hardware Nginx is running? (It matters to a degree for performance, but the general configuration is the same.)

To some extent, this question could also be a software development question, because it's related to software this user had developed, but it's clearly on the sysadmin angle (being about configuring Nginx properly). I think it would be a shame for the ServerFault community to take too strict a view on what's development and what's system administration in most cases. In today's world, many aspects of IT belong to both dev and sysadmin. Having one side pushing the problem onto the other side instead of having a more positive attitude regarding collaboration rarely yields positive results as far as the overall project is concerned.

If the mere mention of "Raspberry Pi" sets you off, this sounds like snobbery to me, sorry.

What's really missing from that question is which OS the system is running. It would be fair to assume that this was running a Debian Wheezy, since that's what recommended by default on the Raspberry Pi, but that should have been in the question.

I don't think there's necessarily anything wrong with asking a question about a Raspberry Pi any more than any other server (whatever its size).

Rather, judging whether the question is on or off topic should be based on what it's actually asking: could this problem apply to a more traditional server, and could this apply to a professional context?

I can't say this particular question was great, but if you re-read and skip the second sentence (the only one that mentions the Raspberry Pi), it certainly becomes a more generic Nginx or Django configuration question:

I've written a basic file server using Django.

When I try to download files from it (multiple large files at once, ~500 Mb each), I can literally download only one or two at once. [...]

Removing the Raspberry Pi reference is indeed one of your suggestions, and this would probably work in this case. Do you then care on which hardware Nginx is running? (It matters to a degree for performance, but the general configuration is the same.)

To some extent, this question could also be a software development question, because it's related to software this user had developed, but it's clearly on the sysadmin angle (being about configuring Nginx properly). I think it would be a shame for the ServerFault community to take too strict a view on what's development and what's system administration in most cases. In today's world, many aspects of IT belong to both dev and sysadmin. Having one side pushing the problem onto the other side instead of having a more positive attitude regarding collaboration rarely yields positive results as far as the overall project is concerned.

If the mere mention of "Raspberry Pi" sets you off, this sounds like snobbery to me, sorry.

What's really missing from that question is which OS the system is running. It would be fair to assume that this was running a Debian Wheezy, since that's what recommended by default on the Raspberry Pi, but that should have been in the question.