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Mehul Kar

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Aug 05, 2021

Terser vs esbuild: Ember app case study

programming frontend javascript

Over the last couple of weeks, I’ve been trying to improve the amount of time my ember apps take to build. The perf-guide in the ember-cli repo is a bit old but the suggestions for how to analyze it still worked for me:

  1. BROCCOLI_VIZ=1 ember build --environment production
  2. Open instrumentation.0.build.json in https://heimdalljs.github.io/heimdalljs-visualizer/#/flame.
  3. Stare at it forever until it starts to make some sense.

I asked coworkers what I’m supposed to do with a flame chart and one pointed me to look at “Self time”. The flame chart in the visualizer doesn’t make it easy to see the nodes that had the largest time.self value, but you can hover over each bar and look for larger values.

flame chart

I noticed that TerserWriter was taking ~20s and thought it might be useful to try out alternatives for minification. Ember CLI’s architecture is pretty confusing, but TerserWriter is located inside broccoli-terser-sourcemap which is a dependency of ember-cli-terser1. Digging into it, I found the line that actually runs the underlying terser tool:

await terser.minify(src, options);


and replaced it with esbuild:

await esbuild.build({
  entryPoints: [inFile],
  bundle: false, // False because the files are already bundled at this point.
  minify: true,
  outfile: outFile,

I had to make some minor adjustments such as removing all the source map things and adjusted debug logs to adapt to the result, but all in all this did produce a production dist/ that looked roughly the same. But TerserWriter‘s self time grew to twice the size to 45 seconds!

One thing that stuck out to me in the esbuild docs was:

The build function runs the esbuild executable in a child process and returns a promise that resolves when the build is complete.

While reading the source for broccoli-terser-sourcemap, I noticed that it also parallelizes each file it feeds to terser. Without any explicit configuration, it defaults to the number of CPUs on the machine (minus one to account for the leader process):

this.concurrency =
  Number(process.env.JOBS) ||
  this.options.concurrency ||
  Math.max(require("os").cpus().length - 1, 1);

// create a worker pool using an external worker script
this.pool = workerpool.pool(path.join(__dirname, "lib", "worker.js"), {
  maxWorkers: this.concurrency,
  workerType: "auto",


I thought it might be worthwhile to try without this parallelization, to reduce the number of factors, so I ran my build with JOBS=1. This time, the numbers were more aligned with my expectations for esbuild (which claims to be 10x-100x faster2).

TerserWriter dropped to 4seconds–5x faster than parallelized terser! Running the original terser implementation with JOBS=1 on the other hand went up to 53 seconds3.

Raw Data

I wanted to see a smaller breakdown of how long it took to minify each file, so I used the logging in broccoli-terser-sourcemap to construct this table:

File Size terser JOBS=1 terser esbuild JOBS=1 esbuild
chunk.0 23.778KB 749ms 151ms 23708ms 7ms
chunk.1 6.625KB 383ms 31ms 23715ms 5ms
chunk.5 7.775KB 416ms 44ms 29765ms 7ms
chunk.6 28.27KB 696ms 74ms 23715ms 7ms
chunk.7 1.805KB 162ms 5ms 14966ms 4ms
chunk.8 2.217KB 181ms 4ms 14927ms 5ms
chunk.9 45.707KB 867ms 138ms 18170ms 8ms
chunk.10 18.248KB 500ms 58ms 26881ms 6ms
ember-fetch/fetch-fastboot 2.736KB 179ms 14ms 14969ms 7ms
auto-import-fastboot 134.432KB 1542ms 750ms 23719ms 125ms
my-app-fastboot 8729.123KB 4808ms 3797ms 3848ms 392ms
my-app 3452.765KB 7899ms 4779ms 6483ms 183ms
vendor 4428.068KB 9941ms 7267ms 9429ms 294ms

If you look closely, JOBS=1 esbuild is the fastest on each individual file, but JOBS=1 terser wasn’t too bad either for all the smaller files–it even bested esbuild in one case! But for larger files, the results were noticeably different. esbuild outperformed terser by 10x.

I’ve decided to come out of the rabbit hole here, but one possible avenue for further exploration would be to investigate how terser works and see if it checks out that it is not an O(N) operation. Another possible way to reduce variables and run each of these 13 files in a deterministic order to see if that changes the results. I was pretty happy to stop here.

Even More Raw Data


assets/chunk.7.19a220231751adc6f9e7.js (1.805KB) -> 1.805KB in 162ms
ember-fetch/fetch-fastboot-e28194c4f1288a25de407aad6f7fab09.js (2.736KB) -> 0.887KB in 179ms
assets/chunk.8.d3bc007e86beb3f59722.js (2.217KB) -> 2.217KB in 181ms
assets/chunk.1.c5058b8350a7f92f058d.js (6.625KB) -> 6.596KB in 383ms
assets/chunk.5.3a7fac165fb83163b87a.js (7.775KB) -> 7.775KB in 416ms
assets/chunk.10.f25d9a8ef2fb4d803961.js (18.248KB) -> 18.101KB in 500ms
assets/chunk.6.229a8539512008cb285f.js (28.27KB) -> 28.219KB in 696ms
assets/chunk.0.988c362969b0fa064219.js (23.778KB) -> 23.63KB in 749ms
assets/chunk.9.a258d4fae07a2ac42ad7.js (45.707KB) -> 45.553KB in 867ms
assets/auto-import-fastboot-ff24d50b4bf7b63ea1c3d7ef54babf53.js (134.432KB) -> 133.896KB in 1542ms
assets/my-app-fastboot-4cdd3aa8b913c28d1628c1be6e682271.js (8729.123KB) -> 8718.29KB in 4808ms
assets/my-app-f629a42d2456316e9837ead6ff7cd404.js (3452.765KB) -> 1616.631KB in 7899ms
assets/vendor-8f243fb0c9b9c8f88fd94a5782813287.js (4428.068KB) -> 1828.842KB in 9941ms


assets/chunk.10.f25d9a8ef2fb4d803961.js (18.248KB) -> 18.272KB in 26881ms
assets/chunk.5.3a7fac165fb83163b87a.js (7.775KB) -> 7.766KB in 29765ms
assets/chunk.0.988c362969b0fa064219.js (23.778KB) -> 23.822KB in 23708ms
assets/chunk.1.c5058b8350a7f92f058d.js (6.625KB) -> 6.602KB in 23715ms
assets/chunk.6.229a8539512008cb285f.js (28.27KB) -> 28.258KB in 23715ms
assets/auto-import-fastboot-ff24d50b4bf7b63ea1c3d7ef54babf53.js (134.432KB) -> 134.748KB in 23719ms
assets/chunk.8.d3bc007e86beb3f59722.js (2.217KB) -> 2.208KB in 14927ms
assets/chunk.7.19a220231751adc6f9e7.js (1.805KB) -> 1.804KB in 14966ms
ember-fetch/fetch-fastboot-e28194c4f1288a25de407aad6f7fab09.js (2.736KB) -> 0.886KB in 14969ms
assets/chunk.9.a258d4fae07a2ac42ad7.js (45.707KB) -> 45.54KB in 18170ms
assets/vendor-8f243fb0c9b9c8f88fd94a5782813287.js (4428.068KB) -> 1880.165KB in 9429ms
assets/my-app-f629a42d2456316e9837ead6ff7cd404.js (3452.765KB) -> 1686.537KB in 6483ms
assets/my-app-fastboot-4cdd3aa8b913c28d1628c1be6e682271.js (8729.123KB) -> 12331.092KB in 3848ms

esbuild (with JOBS=1)

assets/auto-import-fastboot-ff24d50b4bf7b63ea1c3d7ef54babf53.js (134.432KB) -> 134.748KB in 125ms +1ms
assets/chunk.0.988c362969b0fa064219.js (23.778KB) -> 23.822KB in 7ms +0ms
assets/chunk.1.c5058b8350a7f92f058d.js (6.625KB) -> 6.602KB in 5ms +1ms
assets/chunk.10.f25d9a8ef2fb4d803961.js (18.248KB) -> 18.272KB in 6ms +0ms
assets/chunk.5.3a7fac165fb83163b87a.js (7.775KB) -> 7.766KB in 7ms +0ms
assets/chunk.6.229a8539512008cb285f.js (28.27KB) -> 28.258KB in 7ms +0ms
assets/chunk.7.19a220231751adc6f9e7.js (1.805KB) -> 1.804KB in 4ms +0ms
assets/chunk.8.d3bc007e86beb3f59722.js (2.217KB) -> 2.208KB in 5ms +0ms
assets/chunk.9.a258d4fae07a2ac42ad7.js (45.707KB) -> 45.54KB in 8ms +1ms
assets/vendor-8f243fb0c9b9c8f88fd94a5782813287.js (4428.068KB) -> 1880.165KB in 294ms +1ms
assets/my-app-f629a42d2456316e9837ead6ff7cd404.js (3452.765KB) -> 1686.537KB in 183ms +1ms
assets/my-app-fastboot-4cdd3aa8b913c28d1628c1be6e682271.js (8729.123KB) -> 12331.092KB in 392ms +0ms
ember-fetch/fetch-fastboot-e28194c4f1288a25de407aad6f7fab09.js (2.736KB) -> 0.886KB in 7ms +0ms

terser (with JOBS=1)

assets/auto-import-fastboot-ff24d50b4bf7b63ea1c3d7ef54babf53.js (134.432KB) -> 133.896KB in 750ms +0ms
assets/chunk.0.988c362969b0fa064219.js (23.778KB) -> 23.63KB in 151ms +154ms
assets/chunk.1.c5058b8350a7f92f058d.js (6.625KB) -> 6.596KB in 31ms +32ms
assets/chunk.10.f25d9a8ef2fb4d803961.js (18.248KB) -> 18.101KB in 58ms +59ms
assets/chunk.5.3a7fac165fb83163b87a.js (7.775KB) -> 7.775KB in 44ms +45ms
assets/chunk.6.229a8539512008cb285f.js (28.27KB) -> 28.219KB in 74ms +74ms
assets/chunk.7.19a220231751adc6f9e7.js (1.805KB) -> 1.805KB in 5ms +5ms
assets/chunk.8.d3bc007e86beb3f59722.js (2.217KB) -> 2.217KB in 4ms +5ms
assets/chunk.9.a258d4fae07a2ac42ad7.js (45.707KB) -> 45.553KB in 138ms +139ms
assets/vendor-8f243fb0c9b9c8f88fd94a5782813287.js (4428.068KB) -> 1828.842KB in 7267ms +7s
assets/my-app-f629a42d2456316e9837ead6ff7cd404.js (3452.765KB) -> 1616.631KB in 4779ms +5s
assets/my-app-fastboot-4cdd3aa8b913c28d1628c1be6e682271.js (8729.123KB) -> 8718.29KB in 3797ms +4s
ember-fetch/fetch-fastboot-e28194c4f1288a25de407aad6f7fab09.js (2.736KB) -> 0.887KB in 14ms +56ms


  1. I wish ember-cli consumed the broccoli plugin directly instead of wrapping it in yet another package :(.
  2. To be fair, esbuild is meant for bundling and minification, and in this study I’m only using it for minification.
  3. The big caveat here is that I ran all these test builds on my machine and only a couple times to validate that they weren’t flukes. In a more rigorous study, I would want to run the builds more times under different conditions and hardwares and use medians to validate the results.

Want to talk about this blag? Email me or send me a toot @mehulkar!