It's different because it can't just slow down a running task.
Say you use up 150ms of CPU time. Chrome isn't going to let another timer fire for 15s. Then, you're late on a whole bunch of work that needs to get done, so the next timer takes 500ms. Chrome then throttles the next timer by 50s. And so on and so forth...
But if your script had 1% of CPU, that 150ms workload would take 15s to complete. Then the next workload would be late in the same way and take 50s to complete.
It depends on what you're doing. If you split up tasks and interleaves them (such as: process a few items, stop, set another timer, process a few more), actual CPU% throttling would be better. It would be easier to have certain timers take priority, such as heartbeat responses or notifications.
Say you use up 150ms of CPU time. Chrome isn't going to let another timer fire for 15s. Then, you're late on a whole bunch of work that needs to get done, so the next timer takes 500ms. Chrome then throttles the next timer by 50s. And so on and so forth...