Why does Appointment + Keep slowing down then breaking on my site?

I've installed, deleted, and reinstalled a few times now and I keep having the same problem. The appointments feature works smooth and fast and then after a day or so it dramatically slows down the page and then eventually will only load this - Fatal error: Maximum execution time of 30 seconds exceeded in /home/content/16/5607816/html/ls/wp-content/plugins/appointments/appointments.php on line 3900.

The only way I can stop it is deleting the shortcode from my site. I have all the updated Wordpress and your software.

I loved this plugin but this is starting to really get frustrating. Please help!