If you are inspired by Google Instant and other Instant sites and want the same kind of functionality inside Gmail then Cloud Magic fulfils your requirement. It is a browser extension service which indexes your mail data locally on your computer and made it available instantly as you want.
To use this service go to Cloud Magic website and download and install the extension on your browser (download link given at the end).
After installation, login to your Gmail account. You’ll see Cloud Magic search box on the right side. Click on the down arrow given at the side. Select “Manage Accounts” from the given options.
Cloud Magic option panel will open in a new tab. Click on “Click here” to add new account.
Enter your Gmail ID and password. It also supports Google apps account (firstname.lastname@example.org). If you are conscious about your Gmail password and don’t want to give it to the third party app then you have to note that your email/password is stored locally in your system same as mail clients Outlook, Thunderbird, etc.
After you successfully login, cloud magic will start indexing all your mails in your computer locally. It’s is similar to Windows search indexing. You can delete account, go to inbox, add new account using this page.
Now go to your Gmail inbox an search for any mail in the Cloud magic search box given at the top right. You’ll get instant result as you type the query.
To know more about this tool, check out this video.