So it counts idle time. When a process is idle, it doesn't use the CPU, which can be used by other processes.

This basically punishes IO/network bound apps and favors CPU-intensive apps.

I can see why some of the web apps has 100X cost increase since they spin up a process to handle a web request which can take fair amount of network time but uses very little CPU.

The new pricing model is not web-app friendly but favors heavy CPU-intensive backend processing kind of apps.