The essence this when the site has no active users, it is fast as lightning. If your site has more than 10 people ... I turn in chrome ctrl + shift + i -> network parameter and waiting I have very high up to 9 seconds or more, if a person is 20, you can even freeze and prametr wating = 14 or more.
VDS 4000hz 1024 ram debian 7 php 5.4 nginx-1.2.1-2.2 + wheezy2 on,
on command top, there are many processes of "php" which greatly increase the load CPU.
Screen done on site 4 when a person when there are over 15 such processes can be much larger than each of which pulls at least 10%.