The problem with FPS (Assassin's Creed 3)

Faced with such a problem, in General, here is my iron:
Processor - Intel Core i5-4460
Graphics card - AMD Radeon R9 270x
Memory - 4 GB
Windows - Windows 10
Motherboard - GIGABYTE GA-Z97-HD3
In the game, if I look at a lot of particles, such as houses, people, boxes all on one screen, the fps drops to 25-30, if you look at, let's say to water from the ships lying in the port, then a stable 60. The system I like to play powerful 12, and the Witcher 3 on medium-high settings stable 60, in Novigrad and no lag. WHAT'S THE PROBLEM?