I've found "demanding", but I'm not sure if it sounds natural when talking about computers..
You could say that it's CPU-bound.
In computer science, a computer is CPU-bound (or compute-bound) when the time for it to complete a task is determined principally by the speed of the central processor: processor utilization is high, perhaps at 100% usage for many seconds or minutes. Interrupts generated by peripherals may be processed slowly, or indefinitely delayed.
An alternative is resource-intensive.