MNL has a notoriously bad immigration/customs line problem. If you're arriving in the evening (after 7pm), many international flights come in within minutes of each other. Apart from the potentially long wait, there's utter chaos. You're never sure if you're on the correct line. (There are a number of classifications, including tourist, overseas workers, residents, etc..) There are multiple lines for every classification, and you always end up on the slowest line.

Good luck.