Tldr: your build system is so f'd that you have gigs of unused source and hundreds of repeated executions of the same build step. They can fix that. Or, you could, I dunno, fix your build?
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Incremental builds and diff only pulls are not enough in a modern workflow. You either need to keep a fleet of warm builders or you need to store and sync the previous build state to fresh machines.
Games and I'm sure many other types of apps fall into this category of long builds, large assets, and lots of intermediate build files. You don't even need multiple apps in a repo to hit this problem. There's no simple off the shelf solution.
And a fleet of warm builders seems pretty reasonable at that scale.
SourceFS sounds useful for extra smart caching but some of these problems do sound like they're just bad fixable configuration.
It's actually pretty hard. The more builders you have the older the workspace gets and scaling up or cycling machines causes the next builds to be super slow. Game engines end up making central intermediate asset caches like Unreal's UBA or Unity's Cache Server.