I'm having an issue using Woocommerce Bookings plugin.
When I set up a long reservation on a bookable product (let's say, 1 year), the single of this product becomes really slow (~ 30sec. to load, just waiting for server response).
How can I debug this? Any idea how to solve this?
This is most likely connected with the way of getting availability statuses. The slowness is due to extra availability checks due to the combination of how Accommodation and Bookings plugins interact. If you have cleared all stored transients, you can try amending the code and changing the availability verification to be set as hours, not minutes.
Here is a thread with more on the topic https://wordpress.org/support/topic/really-slow-product-page-loading/
Regarding the debugging, try Query Monitor this can be used not only for tracking DB queries.