Suggested by Kurt Hatlevik – New
Each time you load a D365 form som scratch(CTRL-F5), and you take a view in F12, you will see that there are a lot of calls happening, but one of them, that often stands out are App.Css.
At live customers where we see download time of vary from 3s to 12s, and it’s size is approx. 15.9 MB. My experience is that if this file is downloaded slow, then users complain about performance issues, and that the F&O feels “sluggish”.
You can try it out on your own environment by going to :
[Your F&O URL]/WebContent/ApplicationSuite/less/21/0/app.css
See https://kurthatlevik.com/2025/11/23/d365-and-the-performance-of-app-css/ for additional details.
Put bluntly: no enterprise web application in 2025 should ship a 15 MB uncompressed render-blocking CSS file, and F&O is long overdue for a cleanup here. Microsoft can dramatically improve perceived performance across all customers by modernizing static asset delivery for these core UI bundles.
To put the impact into perspective: today a 15.8 MB uncompressed app.css over a typical 15–30 Mbit/s corporate connection costs roughly 4–9 seconds of pure transfer time on every cold load — and that’s before the browser even starts rendering the UI. The same stylesheet, if split and compressed down to ~2 MB of critical CSS, would load in well under a second on the same line speed. With proper client caching on top, most users would pay this cost once per update, not once per session. In other words, a trivial change in how static assets are packaged and cached would turn “wait 5–10 seconds for the client to wake up” into “page is ready almost immediately” for every F&O customer on the planet.