Well, since the "client" is now a browser, the lessons learned regarding performance of the client side executed code of the previous versions are no longer valid. Latency is king, when it comes to a smooth user experience.
Addressing performance starts with the user's experience of the performance of the system and luckily Microsoft have introduced some nice feature where we can start getting some information when the client is in the browser.
The workspace of AX has a URL, no surprise here, and it may look something like this:
But what happens if you add "&debug=develop" after the last character in the URL, like
After the page loads again a small number appears next to the Search box
Left click the number and a performance pane pops up to the right where the stack is presented
You are able to drill-down if you click one on entries in the stack to get more information regarding the elapsed time
So, a short but effective start off to digging into the end-users performance challenge simply by adding a small snippet after the URL.