I'm having very slow performance with the plugin, and this was one reason I thought it might be happening. I'm getting this slow error:
slow_errors_common: /www/wp-content/plugins/appointments/includes/class-app-gcal.php:34 __construct() /www/wp-content/plugins/appointments/appointments.php:219 get_gcal_api()
The plugin works as long as I'm looking at the current month, for certain time periods, but as soon as I select longer time periods and advance to the next month, it times out. I'll give you an example:
If you go to "Reserve a Studio", you will get 3 calendars for the 3 studios. If you select a time of "3 hours" from the dropdown, it will take a while to refresh the calendars, but it might work. If you then click "Next Month" you will get a timeout.
My host ran that test and then emailed me the slow error which referred to the appointments plugin and gcal.