One weird trick to reduce Eleventy Image Build Times by 60%

This web site does a fair bit of built-time Image Optimization using the HTML Transform method provided by the Eleventy Image plugin. I took a bit of a set-it-and-forget-it approach with the plugin on this site, mostly for my own convenience. My usage looked something like this:

import { eleventyImageTransformPlugin } from "@11ty/eleventy-img";

export default function(eleventyConfig) {
	eleventyConfig.addPlugin(eleventyImageTransformPlugin, {
		failOnError: false,
		formats: ["svg", "avif", "jpeg"],
		svgShortCircuit: true,
	});
};

Eleventy Image includes a fair number of caches and performance optimizations to make this default behavior pretty fast.

• It uses a memory cache to de-duplicates multiple requests to optimize the same image.
• It uses an on-request image processing engine to avoid processing images during local development.
• It uses a hash for output file name and checks the target file location to avoid reprocessing images.

The biggest drawback with the third method is that when you’re building on a deployment server, these environments start with an empty output folder (and thus, an empty disk cache). What can we do to re-use the disk cache and avoid reprocessing unchanged images on each new deploy?

With just a few lines of configuration code to create an intermediate output folder in a location that is persisted between builds (.cache), I was able to drop my site’s build time from an embarrasing 9:40 down to a respectable 3:56 (for throughput info, the output folder has 7,779 files weighing 492.7 MB).

I think this approach is very reusable and we’ll likely bundle it into a future version of Eleventy Image. Until then, you can use it yourself by adding the following lines of configuration:

+import fs from "node:fs";
 import { eleventyImageTransformPlugin } from "@11ty/eleventy-img";

 export default function(eleventyConfig) {
 	eleventyConfig.addPlugin(eleventyImageTransformPlugin, {
+		urlPath: "/img/built/",
+		outputDir: ".cache/@11ty/img/",
 		failOnError: false,
 		formats: ["svg", "avif", "jpeg"],
 		svgShortCircuit: true,
 	});

+	if(process.env.ELEVENTY_RUN_MODE === "build") {
+		eleventyConfig.on("eleventy.after", () => {
+			fs.cpSync(".cache/@11ty/img/", path.join(eleventyConfig.directories.output, "img/built/"), {
+				recursive: true
+			});
+		});
+	}
 };

You may also need to persist .cache on your web host. This behavior is provided for-free if you’re using Vercel or Cloudflare Pages.

Subscribe to github.com/11ty/eleventy-img/issues/285 for future updates.