As someone who built Changi Airport as realistically as I could with the game’s engine, at some point in the game you start being limited by the game itself rather than your planning. It does start getting frustrating when things start to slow and tank. The airports in real life are designed like that exactly because the design works, but only when real humans operate it. In my megabuild I had many ai limitations such as planes waiting forever to taxi because of the queue system, passengers dropping off at the wrong terminal, catering trucks taking a scenic route around the whole airport to a depot on the other end instead of the nearby depot.
However I don’t want it to discourage you and I wish you all the best! Just know that if the airport cripples, it’s not always your fault and you should be brave to close a stand or two or ten to make a tweak to systems and designs