There should be very little difference between get_field() and get_post_meta() for performance in most cases. There are some exceptions to this. It depends on the field and what it is set to return, for example gallery fields that return arrays of image data. ACF function are, for the most part, wrappers for WP functions.
The main difference with using get_post_meta() is that you can deactivate ACF in the admin and the front end of your site will continue to function until you reactivate.
