I have a big project with data files, loads of includes and sass, but my project takes 12-15 seconds to reload is there a way to only get jekyll to cache pages your not working on etc.
Have you tried using incremental builds?
Enable the experimental incremental build feature. Incremental build only re-builds posts and pages that have changed, resulting in significant performance improvements for large sites, but may also break site generation in certain cases.
jekyll build --incremental
Is this instead of Jekyll serve while working on it
No not exactly. You can use the --incremental flag with jekyll serve too. Or you can enable it for both build and serve by adding incremental: true to your _config.yml file.
You may experience strange things using it though. Because of the way it caches files things don’t always behave how you’d expect. If you have a fairly simple site you’ll probably be OK.