Hi Mahesh,
This link might be useful to you:
If the usage is an issue, when you are clicking on Inbox folder, have you checked the performance by logging to your server? It might possible that when you are opening your Mailbox, it connects to server and getting stuck due to CPU/RAM.
Pratik Jajal
Younited Host - The best Web Hosting Provider
Email: pedro@younitedhost.com