Hi Mohsin,
Are you talking about the Discover page loading time? That might be impacted by how large each doc is. There's ways to eliminating the loading of some field if it's really large.
What browser are you using? Internet Explorer can perform much slower than Chrome on some tasks in Kibana.
And the other thing you can check is the stats by clicking the little arrow at the bottom of the Discover Histogram, then the Statistics button.
Regards,
Lee