2K has no official designation and is sometimes used to describe 1440p.

The higher the resolution you run a game at the more likely it is that the GPU becomes the bottleneck for the frame rate.

Generally GPU load scales with resolution and graphical fidelity, while CPU load mostly just scales with framerate irrespective of resolution or graphics settings - so you might be CPU bottlenecked at max settings 1080p with an average CPU and a mid-high end GPU, but even with the current highest end GPUs and a mid range CPU you're likely not bottlenecked by the CPU at 1440p or 4k because the GPU isn't pushing out as many frames.

Nearly the same thing, give or take a few percent. (https://en.wikipedia.org/wiki/2K_resolution)

78% more pixels at 2560x1440, also the performance sweet spot for high end GPUs.

2560x1440 is often called "2k"

Nothing, AFAIK