Chasing build speed on Jekyll 4

I’ve been putting off upgrading from Jekyll 3.10. Partly late-adopter habit, partly because my setup worked fine, and a version bump seemed simple enough. I should know better by now. It mostly was. I run Jekyll on my NAS, in Docker, with a Gemfile shared by the GitHub Pages deploy. I don’t use Jekyll’s incremental build locally, so every time I saved a post I was writing, Jekyll did a full rebuild before I could preview the change. This year that’s added up, the best part of 90 new posts and the site’s up to 360 files for Jekyll to churn through each time. That wait had crept up to 46 seconds, long enough that I’d lost the thread of what I was writing by the time the page reloaded. The Gemfile’s jekyll version went from "~> 3.10.0" to "~> 4.3" (resolving to 4.4.1), and the Dockerfile’s base image went from ruby:3.2-slim to ruby:3.4-slim to match what the deploy workflow already uses. Since moving to GitHub Actions, the deploy builds the site itself from that same Gemfile, so bumping…