After experimenting and measuring, I've found out some numbers:     static func maxNumberOfSimultaneousThreads(device : MTLDevice) -> Int? {         switch device.name {         case "AMD Radeon Pro Vega 64": return 8192         case "AMD Radeon RX Vega 56": return 4096         case "AMD Radeon Pro Vega 20": return 4096         case "AMD Radeon R9 M370X": return 2048         case "Apple A8 GPU": return 512         case "Apple A9 GPU": return 1024         case "Apple A10 GPU": return 1024         case "Apple A11 GPU": return 1024         case "Apple A12 GPU": return 4096         case "Apple A12X GPU": return 2048         default: return nil         }     } I've measured these numbers with a dummy compute shader that uses about 4K bytes thread local memory. What it means is that for example on my iPhone XS Max (Apple A12 GPU), running the compute shader once takes as much time as running the compute shader up to 4096 times; running it more times will take at least double that run time. Not sure how reliable these numbers are, so ymmv ...

It seems that the answer to this is more complicated than I thought. It not only depends on the specific kind of machine it is running on, but also on the code itself, specifically how much local thread memory the compute shader uses.

Well, it seems Apple agreed that modularity is not UIKit's strength, and invented SwiftUI :-)

Yes, I found now Apple documentation for that use case. You can do that via let folderPicker = UIDocumentPickerViewController(documentTypes: [kUTTypeFolder as String], in: .open)