1) It's normal for Kiro (and almost every AI editor) to use a lot more CPU when you first start it up, because it is indexing your codebase in the background, for faster and more accurate results when you prompt. That indexing should complete at some point
2) On initial setup of Kiro it will import and install your plugins from VS Code. If you have a large number of plugins this continues in the background, and can be quite CPU heavy as it extracts and runs the installs for each plugin. This is a one time performance hit though.
3) If your computer is truly idle, most modern CPU's get throttled back to save power. When the CPU is throttled, even a tiny amount of CPU utilization can show up as a large percentage of the CPU, but that's just because the CPU has been throttled back to a very slow clock speed.
In my setup (minimal plugins, medium sized codebase, computer set to never idle the processor clock) I rarely see Kiro helper go above .4% CPU utilization, so if you are seeing high CPU it is likely for one of the above reasons.