Page dependent ajax function

Hi,

I have different functions defined in functions.php which are required in different page. So, I was wondering should I use add_action() for all the functions directly or use if() condition to call based on page slug.

For example, let say I have 10 functions defined in function.php which are called from 10 different pages. So, should I code like below (where I used if condition), or just add_action for all functions irrespective?

Just from performance point of view.

<?php
function sendMail(){
   .....................
   .....................
    wp_send_json($myvar);
}
if(is_page('contac-us'){
    add_action('wp_ajax_mf_sendMail', 'sendMail');
    add_action('wp_ajax_nopriv_mf_sendMail', 'sendMail');
}
?>