Closing other programs, tools, addons might help. Having a powerful CPU with fast cores, might help, but there will be the point your PC is fast, everything is optimized, and you still have sometimes those lags. And many hints and tips relating to shooters or other action-games do not really work.
but I don’t understand why GPU had to take burden of memory instead CPU.
ignore the memory-part...it's mainly the utilization of CPU and GPU
Operations on the GPU-side are highly parallelized....those on the CPU-side are not.
If your GPU can't handle the game well, you have just less frames.
But, on the CPU things sometimes can't be done at the same time, cause the next operation might be depend from that now.
( It depens on the game engine and how things are done)
And planet zoo is a complex game. Objects are not only something that is visible, like in many other games, they influence other parts (traversable area and so on)
If there is a delay on the CPU-side (for whatever circumstances....even with the best hardware) , the game can’t progress....and your GPU has to wait and this is that freeze
Very simplified: the GPU is the lawnmower. If the grass is higher it might run slower. The CPU is the person that operates the lawnmower and has sometimes a bit hassle to turn around on one end.