to answer the question in the title, i think that's the time it took to generate those static pages

your /contacts took 3.2s to generate during the build time

I see, thanks for the respond

but why some pages have the brackets and some and dont?

is it only pages that have long loading time shows the bracket?

if you meant the square brackets [] then those pages are just dynamic routes

sorry, I mean the parentheses

/contacts/[contact_id] (853ms) means:
* /contacts/[contact_id]/page.tsx is being built
* that build takes 853ms

yes only pages with expectedly slow generation time display that

generally it's not a problem since it only impacts the build time

runtime users are not impacted

I see, from the numbers shown in the screenshot, are the size of the pages ok? when I run npm run dev in localhost:3000 while navigating between some pages, it is slow, but when I did npm run build and npm run start or next start the navigation of the pages loads instantly

they are red so they are not "fully ok", but imo 200kB is fine enough. you should still reduce it if you can, but if you can't then 200kB is not the end of the world.

npm run dev is expected to be slow, because (1) the pages aren't optimised and (2) all pages are built on-demand

npm run start is much faster because (1) everything is optimised and (2) all pages are built at build time, ready to serve. you should always run npm run start for applications facing the user. only use npm run dev when you develop the web app