If your site has a large number of posts, it’s likely that your biggest time saving would be made by processing less of them at development time. You can do this with the limit_posts command line option (https://jekyllrb.com/docs/configuration/options/#build-command-options): $ bundle exec jekyll build —limit_posts 5
For general build time improvements, I’d highly recommend profiling your site to find the best place to optimise: $ bundle exec jekyll build —profile