check-in and boarding time
 

Last week I flew a domestic flight from Fujian to Taiwan - the check-in counter opened 1 hour and closed 30 minutes before departure, and if online check-in was done, I could arrive the security as late as 20 minutes before departure. The boarding only started 15 minutes before departure.

However, on international flights, the check-in closed 1 hour before departure, boarding started 40 minutes and ended 20 minutes - this is the time where I was just entering the airport if taking a domestic flight!

Why is there so much time wasted for international flight?! Can't the airport make them operate in the same way as domestic where I can check-in until 30 minutes before departure?

Apples and oranges.

Probably because more time is involved in checking passports and ensuring passengers have the proper paperwork to travel on international flights?

Out of curiosity, do both China and Taiwan consider Cross-Straits flights to be domestic?

International flights would require passport checking and such. They may also use larger planes that take longer to board (especially if one end of the domestic flight is on a small island like Kinmen or Matsu, the ROC controlled parts of Fujian).

The cross-strait flights from Kinmen and Matsu to the island of Taiwan or Penghu are domestic - they are the only flights which are permitted to directly overfly the strait across the median.