Yes and no. In some cases (3rd party content that you don't control), there's no way to know what script/iframe is going to do/use. For example, ads script that uses `document.write()` and kills perf.

Turning on certain policies can also help you be certain quality/standards are being met across your site. For example, the creative team forgets to optimize images and you end up shipping many more MBs of images than actually needed. The `image-compression` feature policy could be used as a gut check to make sure that never happens.