FID measures how fast a web page reacts whenever it loads information inputted by the user. I.e FID is about the clicks and button presses made by the users.
According to FastComet The owners of the webpage should aim for FID to be less than a 100 milliseconds in order to create a comfortable environment.
It is important to note that measuring FID is not the easiest thing in the world as it required practical testing, i.e to be useful tests have to be conducted on actual users. Which means your score might be a result of a mix of confounding variables. E.g the internet speed of the user or their hardware.
How do we improve on slow reaction times?
Considering the fact that long FIDs are caused by unorganized downloads of scripts and pictures, improving it will depend on exactly this. It is imperative that we present all of this in a careful and organized manner to the browser.
This is done by using HTML in order to control how the scripts are downloaded, optimizing pictures and getting rid of unnecessary scripts.
The aim should be to optimize the downloaded elements in order to get rid of pause-and-start problems on disorganized web pages.
Why brothers freeze
A browser is a software that fulfil tasks in order to present a specific page. More concretely - these tasks consist in downloading code, pictures, fonts, stylistic information, scripts and making them work according to HTML instructions. This complex process is called rendering. The word render means "making" and this is exactly the function of the browser; ordering code and pictures (among other things) to "make" a whole web page.
Individual rendering tasks are called threads (threads of execution). As in the individual part of the whole process. One of the most important threads in a browser is called the "Main Thread" which is responsible for the entire web page coming together and the user seeing it on demand. A good analogy of the Main Thread would be a highway where instead of cars we get pictures and scripts that are activated on request.
If one of the pieces of code is too big or too slow it bring other tasks down with it; the latter have to wait until the former get off the highway (as in the task is completed)
Developers should pay attention to which code is downloaded first. As well the ranking that the activation follows. This type of approach will help the web page load faster.
It is important to note that FID scores vary and are not particularly consistent. The score changes from user to user. This happens naturally as different users use differing CTA-s while on the website. Some even wait a little bit before making a decision which helps the elements load in and get activated.