e.g. we generate those static pages

and each page runs getBySlug

and after 700 pages backend timeouts bc of too many requests

So I need to set up a delay before each page generation but I cant find an option to do so

If it goes one by one, maybe make a delay on your backend to respond after for example 3 seconds

Nah thats not an option

btw I am not sure if next does it one by one

I've added 3s timeout after fetching a single page but logs didnt look like each page was generated for 3 seconds

i think the page generation is parallelised. which makes it quite hard for things like this.

since you can access the backend code, i think you should do this:
* set up a rate limiter in the backend, so it doesn't crash.
* in the nextjs build, just loop if the request fails due to the rate limiter. maybe stop looping after the 10th iteration.