Why React dynamic import file (~3.7 KB) take 650 ms on first load but only 45 ms on refresh with AWS S3 + CloudFront?

I have a React app deployed on AWS S3 + CloudFront.

One of my routes uses a dynamic import (import()), and the corresponding JS chunk is very small (~3.7 KB).

On the first load, the request for this file takes about 650 ms, but if I refresh the page …


This content originally appeared on DEV Community and was authored by Sohan Ram

I have a React app deployed on AWS S3 + CloudFront.

One of my routes uses a dynamic import (import()), and the corresponding JS chunk is very small (~3.7 KB).

On the first load, the request for this file takes about 650 ms, but if I refresh the page immediately afterward, it only takes about 45 ms.

Here’s the request and response details from Chrome DevTools for the file:

Request URL: https://admin.simprosysapis.com/assets/RolesList-DVgRFD0k.js
Request Method: GET
Status Code: 200 OK
Remote Address: 18.66.57.77:443
Referrer Policy: strict-origin-when-cross-origin

Response headers 

cache-control: max-age=3600, must-revalidate
content-encoding: br
content-type: text/javascript
server: SimprosysAPI
via: 1.1 ...cloudfront.net (CloudFront)
x-cache: Miss from cloudfront

Request headers 

accept-encoding: gzip, deflate, br, zstd
cache-control: no-cache
pragma: no-cache
user-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) ...

My questions

  1. Why does a 3.7 KB file take ~650 ms to load the first time?
  • Is this mostly TLS/handshake + CloudFront latency rather than file size?
  • Is the x-cache: Miss from cloudfront header responsible?
  1. Why does it become so much faster (~45 ms) on refresh?
  • Is the browser using disk/memory cache or a CloudFront cached copy?
  1. Is there a way to improve the first load time for these small dynamic chunks (e.g., preloading, prefetching, or CloudFront settings)?

What I tried

  • Checked headers: content-encoding: br confirms compression is working.
  • Cache headers look reasonable (max-age=3600).
  • Behavior is consistent across multiple builds.

Expected / Desired

I want to understand if this latency difference is normal for CloudFront + S3 dynamic imports, and whether I can optimize the first-load latency further.


This content originally appeared on DEV Community and was authored by Sohan Ram


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Sohan Ram | Sciencx (2025-10-01T11:18:36+00:00) Why React dynamic import file (~3.7 KB) take 650 ms on first load but only 45 ms on refresh with AWS S3 + CloudFront?. Retrieved from https://www.scien.cx/2025/10/01/why-react-dynamic-import-file-3-7-kb-take-650-ms-on-first-load-but-only-45-ms-on-refresh-with-aws-s3-cloudfront/

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" » Why React dynamic import file (~3.7 KB) take 650 ms on first load but only 45 ms on refresh with AWS S3 + CloudFront?." Sohan Ram | Sciencx - Wednesday October 1, 2025, https://www.scien.cx/2025/10/01/why-react-dynamic-import-file-3-7-kb-take-650-ms-on-first-load-but-only-45-ms-on-refresh-with-aws-s3-cloudfront/
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Sohan Ram | Sciencx Wednesday October 1, 2025 » Why React dynamic import file (~3.7 KB) take 650 ms on first load but only 45 ms on refresh with AWS S3 + CloudFront?., viewed ,<https://www.scien.cx/2025/10/01/why-react-dynamic-import-file-3-7-kb-take-650-ms-on-first-load-but-only-45-ms-on-refresh-with-aws-s3-cloudfront/>
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Sohan Ram | Sciencx - » Why React dynamic import file (~3.7 KB) take 650 ms on first load but only 45 ms on refresh with AWS S3 + CloudFront?. [Internet]. [Accessed ]. Available from: https://www.scien.cx/2025/10/01/why-react-dynamic-import-file-3-7-kb-take-650-ms-on-first-load-but-only-45-ms-on-refresh-with-aws-s3-cloudfront/
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" » Why React dynamic import file (~3.7 KB) take 650 ms on first load but only 45 ms on refresh with AWS S3 + CloudFront?." Sohan Ram | Sciencx - Accessed . https://www.scien.cx/2025/10/01/why-react-dynamic-import-file-3-7-kb-take-650-ms-on-first-load-but-only-45-ms-on-refresh-with-aws-s3-cloudfront/
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" » Why React dynamic import file (~3.7 KB) take 650 ms on first load but only 45 ms on refresh with AWS S3 + CloudFront?." Sohan Ram | Sciencx [Online]. Available: https://www.scien.cx/2025/10/01/why-react-dynamic-import-file-3-7-kb-take-650-ms-on-first-load-but-only-45-ms-on-refresh-with-aws-s3-cloudfront/. [Accessed: ]
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» Why React dynamic import file (~3.7 KB) take 650 ms on first load but only 45 ms on refresh with AWS S3 + CloudFront? | Sohan Ram | Sciencx | https://www.scien.cx/2025/10/01/why-react-dynamic-import-file-3-7-kb-take-650-ms-on-first-load-but-only-45-ms-on-refresh-with-aws-s3-cloudfront/ |

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