Server tick will increase up to 400ms or more during night especially, thus we monitored the java program using YourKit Java Profiler and noticed that it may be caused by the AI program of hostile mobs (in ayo.class, since obfuscated, and this is only a guess).

Further tests are done on other mobs like large amount (~50) of chickens, cows, etc. And the server won't lag during the day.

The attached screenshot records several seconds when the avg tick ~ 300ms or more during night.