I run a site with practice exams for Dutch certification exams. Next to the exams there are guides: how buoys work for the small boating licence, which sound signals you need to know, how to book the exam. On 24 October a deploy failed, and the cause was a function that every one of those pages called without anyone thinking about it.

Here's what the build log said, what was actually slow, and the rule we wrote down afterwards.

What the build log said

At the time we were on Vercel's Hobby plan. The log listed the build machine as "2 cores, 8 GB", and Next.js 16.4.2 generated the static pages with a single worker:

Generating static pages using 1 worker (0/187) ... Failed to build /(nl)/(site)/klein-vaarbewijs-1/betonning/page: /klein-vaarbewijs-1/betonning (attempt 1 of 3) because it took more than 60 seconds. Retrying again shortly. ... Failed to build /(nl)/(site)/klein-vaarbewijs-1/bpr-en-rpr/page: /klein-vaarbewijs-1/bpr-en-rpr after 3 attempts.…