If you are getting the following error message:
2015/07/03 14:58:13 [error] 949#0: *236 upstream timed out (110: Connection timed out) while reading upstream, client: xx.xxx.xx.xxx, server: your.server.com, request: "POST
/wp-admin/admin.php?import=wordpress&step=2 HTTP/1.1", upstream: "http://xx.xxx.xx.xx.xx/wp-admin/admin.php?import=wordpress&step=2", host: "your.server.com", refer
This is because the file you're trying to import is to big to handle in the upstream.
You can fix this by adding/changing proxy_read_timeout in your nginx config.