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.