How to reduce uncached page generation time?

there is no simple general answer for this, as it depends on what is causing the issue.

Best way is to check your page using some Insights tool:

They will tell you where the issue is.

Big images are commonly a big issue – there are many converters to WebP format, that will lower load speed.

You can try to load only JS and CSS files that are required on certain page.

If you have some big operations in PHP executed before page is loaded, you can try to reduce those.

But as I said, make the metrics check, and they will tell you what you need to correct.