Speed up Next.js build

[mjq-sentry]

The full site Next.js build currently takes ~38 minutes in Vercel, which is annoyingly slow. I previously got it down to ~18 minutes in Vercel but it regressed to over 35 minutes in c8d5391. Look into want happened and see how much time we can get back.

[stephanie-anderson]

Optimisations are done for now, closing issue