Job listing loading very slow

This can be affected by many factors;

1. Number of postings
2. Server power
3. Other plugins installed
4. Theme
5. Heartbeat API if running on an underpowered server (http://www.inmotionhosting.com/support/website/wordpress/heartbeat-ajax-php-usage)

Can be resolved by optimising your setup, installing a caching plugin (w3 super cache) and general debugging.

p.s. are you using https://wordpress.org/plugins/wp-job-manager/ or https://wordpress.org/plugins/job-manager/ ? I see you have posts in both forums.