In terms of project scale, doctrine vs zend-db-table speed and performance, when should I use doctrine inside Zend project, and when zend-db-table?
Any ORM framework gives you benefit for development productivity, not runtime efficiency. Doctrine is no different from Zend_Db_Table in this regard.
If you are choosing between Doctrine and Zend_Db_Table, choose based on the features that make it easier or faster to write the code.
No ORM framework can automatically make database queries faster in the general case. If you need high-performance database queries, you should learn to code SQL queries, and design your schema and indexes to support performance given the queries you need to run.