is Form api ajax attribute good in performance ? does it require an entire bootsrap?
Whether or not the request method is AJAX has no bearing on whether a full bootstrap is performed - that's completely down to the behaviour of the path that's being requested. If your AJAX call requests '/js/my-page.js', and that menu path is somehow configured not to bootstrap fully, then you won't incur a bootstrap penalty.
To that end, I would highly recommend installing the High-performance JavaScript callback handler module.

JavaScript callback handler is an interim solution for high-performance server requests including (but not limited to) AHAH, AJAX, JSON, XML, etc.
This project targets module developers and provides a &quot;bare bone&quot; callback handler which is intended to be addressed by modules wanting to improve response times for specialized tasks.

It will allow you to define router items for which a full bootstrap will not run; in fact you can pick exactly which modules should be loaded, and which other files should be included, for your callback's requirements.
I've used it before with great success, and have seen very noticeable improvements when converting custom (and even core) AJAX callbacks to use this method. It's blindingly fast.
It also has integration with a bunch of other modules (SHS for example).
As to whether Drupal will be able to handle your project - that's not for us to say; have a look at the various options available (including this one), do some benchmarking and stress testing per your project/client-established requirements, and it'll become apparent whether or not Drupal will meet your needs.