It's hard to give suggestions without more information.

My guess is that it's either a GPU problem or a CPU one. The game is quite demanding on the CPU, since all those bugs do multi-threaded crowd simulation.

GPU-wise, it pre-renders the maps to textures, so texure RAM is an issue sometimes. If you have an integrated Intel GPU, it might just be using the system RAM for textures.

That said, I used my 6-year old MacBook as a min-spec machine, so it's odd that the game would be slow on modern hardware.

To test out what the slowdown is, try these things:

1) Pause the game. If it runs fast when paused then the CPU is having trouble.
2) Turn down the World Detail to LOW - if this helps, it's a texture memory problem
3) Turn down the particles & splatter to minimum - this is a posible GPU rendering bottleneck
4) Turn off shaders - Another GPU possibility