A lot of requests time out to a node server while piping large files

I'm piping a lot of big files to users and it works well. Sometimes there are 5-10 current users downloading (I don't know the exact value) that a lot of requests aren't recieved (I checked that client-side), and even the console.log won't be fired.

Does this happen because of the eventual network load (which I assume) or is it maybe a limit (e.g. max 10 connections for the /download route) defined by the express module?

Example:

app.get("/download", funtion(req, res) {
    console.log("start"); // won't be logged
    // ...
    obj.pipe(res);
    // ...        
});