So reducing this would be a multi-year project of a sizeable team (let's say 5-10 people). How do you get someone to sign off for this kind of expediture and not lay everyone off when the next economic fart comes?
Because THAT'S the hard problem to solve in any bigger organization. Not hacking the code.
>So reducing this would be a multi-year project of a sizeable team
Maybe, or just a knowledgable person to dig in for a bit and solve it. On one of my teams when I was back at Microsoft was working on a similarly huge codebase with build times twice as long. We had a senior engineer who was really into build optimizations from a previous job who wrote a tech design, implemented better project splitting in our monorepo, better local caching, and finally cloud caching, all within 3 months. When I left, our builds ranged from 30 seconds to 3 minutes.
It's not guaranteed that it'll be this simple, but I also don't think it's guaranteed it'll be 3 years and several people.
To clarify: when I say value I don't just mean monetary value - though of course everything in a business ultimately does boil down to that, including the total expectation of risks.
Long build times are an operational risk - it's very hard to apply patches to solve incidents with a 20 minute build time.