503 error means your server is overloaded. Usually it happens on limited shared hosting, when CPU and/or memory is limited by the hosting provider.
To avoid these issues 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 you still get 503 errors, you may should decrease prebuild speed. You can configure it in Settings > Caching > Warmup > Prebuild speed.