The "OSError: write error" message appears in the log when the client disconnects before the response has been sent to them.
If you're getting the "Something went wrong" page, the first thing to do is check your error logs for tracebacks that you can use to debug the problem.
For the speed part - Your web app has one worker. If it's busy handling a request, then incoming requests will have to wait in a queue before they are handled. If you have one or more views that take a long time, then they may hold up the worker and prevent other views from responding quickly. Check your access logs - there's a field in there that shows how long a request took to process (it's called response-time). You can use that to determine what your web app is doing.