This happens only in China, where the internet is "walled-off" and specifically most of the google services are completely blocked.

1) Open browser, go to http://docs.mongodb.org/manual
2) Observe the page loads after 1 minute or so
3) Wait for a couple of minutes and observe search box does not appear

Expected:
1) Page load should be faster. Most of time was spent trying to resolve google fonts:

css?family=Inconsolata
fonts.googleapis.com
GET (failed)
net::ERR_CONNECTION_TIMED_OUT
docs.mongodb.org/:7
Parser
0 B
0 B
34.29 s
-

2) Search box should appear

Work around is to use VPN which solves these problems. But VPN is a luxury option available only to a small number of users.