Hi Naren,
We need to identify the script that causes this slow running issue.
Please use the Developer Tools (if on IE8, 9) or any other tool to see what script runs slow.
Then on the form try to remove all fields and render the page. Then you can add fields one by one and see if a particular field generates the issue.
Also try to use Chrome and Mozilla FireFox.
PS
What's your QWP and SP version?