What happens when you run `terraform apply`? Arguably, a lot of things, but at its core it:
- Computes a list of resources and their expected state (where computation is generally proportional to the number of resources).
- Synchronizes the remote state by looking up each of these resources (where network ingress/egress is proportional to the number of resources).
- Compares the expected state to the remote state (again, where computation is generally proportional to the number of resources).
- Executes API calls to make the remote state match the expected state (again, where network ingress/egress is proportional to the number of resources).
- Stores the new state (where space is most certainly proportional to the number of resources)
This is a bit simplified, but my point is that in each of the five operations, the number of resources can be used as a predictor for the consumed compute resources (network/cpu/memory/disk). A customer with 10k resources is necessarily going to consume more compute resources than one with 10 resources.