I just tested posting the JSON to this app:
import vibe.d;
static this()
{
void handle(HttpServerRequest req, HttpServerResponse res)
{
static int cnt;
res.writeBody(to!string(cnt++)~"\n"~req.json.toString()~"\n", "text/plain");
}
auto settings = new HttpServerSettings;
settings.port = 8080;
listenHttp(settings, &handle);
}
and the response is instantaneous. Is there anything else happening in your app? If not, does this simple app also show the delay for you?