that was already in my package manifest.

yes, that was helpful. Especially specifying which bundle. That however opens a bigger can of worms. ".module" often doesn't work. The way I expose Bundles of Packages to other packages is with this handy fix: https://forums.swift.org/t/swift-5-3-spm-resources-in-tests-uses-wrong-bundle-path/37051/36 and https://developer.apple.com/forums/thread/664295 Then you create a public extension on Bundle. All this still doesn't fully solve the task of one Module that 'vends' Strings.

I'm having the same issue. I thought it might have been related to concurrency, but even calling this on a worker queue results in the video output freezing until this method 'startWriting()' completes. For me it takes about 2 seconds.