is proxy_read_timeout effects for a slow response?

[foxgab]

if nginx connect to a proxied server successfully, but the server takes a
long time before starting to send the response, will proxy_read_timeout
directive effect? if not, is there any other directive do that timeout kind
work?
when proxy_next_upstream is set to "off", which status code will be respond
if timeout?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,276066,276066#msg-276066