Usually it happens on small VPS, or limited shared hosting, when CPU is limited by the hosting provider.

To avoid it you should do the following steps:

  • Enable Compute API
  • Enable Prebuild Booster

These options will speed up optimizing process, and also will decrease CPU usage.

If your site is running on shared hosting they usually limits the CPU usage (CPU time). That means they won’t let you use the shared CPU all the time.

100% CPU usage is temporary, it happens while Swift generates the cache, but once it is generated CPU usage will be about 0%.
However there is an easy solution to keep CPU usage under the limit, you can limit Prebuild speed. If you set it to Moderate or Reduced the plugin will wait a bit between preloading pages, it will decrease the CPU usage.
You can configure it in Settings > Caching > Warmup > Prebuild speed.