Adding New post is very slow

If I add a New post to wordpress blog when I click on publish it shows loading for few mins and then show timeout. (then the server is loaded)

When I check the status of the post, its been published in few seconds, but I never get the page reloading saying the post is published.

Then If I add one more post after this then the next post is getting published fast (normal time)

This issue is only for publishing the first post after an specific interval. From there on all get published very fast. any fix for the same?