Need Elastic Search Architects to help with design issues -- 2

We have implemented Elastic Search to be able to search through emails and attachments. We have hit a few roadblocks as part of the design limitation for which we need your support to understand how to solve.

E.g:
We have indexed email and attachment objects separately. We have established a parent child relationship through field mapping. If we need to search for a keyword which may be present in an email or at attachment of the same email, we are unable to do that.

We also have lot of performance issues and we need help to optimize that