The way files are served with roundup means they are
read in into memory as a whole, before being sent to
a client using write().
This becomes impractical for large files.
On Linux (and similar), a call to sendfile() seems
more appropriate. Even when not available, the data
could be streamed instead, i.e. processed in chunks,
instead of as a whole.

(Sorry, I had forgotten that I already file this one, and created a new one.
As the new one is more specific (I have a patch that I may submit if the idea is accepted), I'll mark this one as a duplicate for 1855259)