Swift async Task captures value too long - swift

Question

I want to do some async processing in my MTKView's draw(_:) method. For that I'm first getting the currentDrawable and then using it inside of a Task:
override func draw(_ rect: CGRect) {
guard let currentDrawable = self.currentDrawable else { return }
Task {
// <image filtering>
let destination = CIRenderDestination(width: Int(drawableSize.width),
height: Int(drawableSize.height),
pixelFormat: self.colorPixelFormat,
commandBuffer: nil,
mtlTextureProvider: { () -> MTLTexture in
return currentDrawable.texture
})
try await asynContext.startTask(toClear: destination)
try await asynContext.startTask(toRender: image, to: destination)
currentDrawable.present()
}
}
While this works, the performance is much worse than using a DispatchQueue or staying on the main thread.
It seems the Task is holding on to the drawable for much longer than its actual execution, which causes the next call to currentDrawable to stall when no more drawables are in the pool at the moment.
I assume the Task itself is still referenced/used by the concurrency runtime after its execution for a little while longer.
Is there a way to prevent that? Or, alternatively, to have the Task's operation block release the drawable early?
As by matt's request, a little more elaboration:
I want to move some computation Core Image is doing on the filter graph into a background queue to not block the main thread. For that, I created a little helper actor:
actor AsyncContext {
private let context: CIContext
init(context: CIContext) {
self.context = context
}
#discardableResult
func startTask(toClear destination: CIRenderDestination) throws -> CIRenderTask {
return try self.context.startTask(toClear: destination)
}
#discardableResult
func startTask(toRender image: CIImage, to destination: CIRenderDestination) throws -> CIRenderTask {
return try self.context.startTask(toRender: image, to: destination)
}
}
The actor should ensure that calls to startTask(...) are happening in a background queue (which they do).
I'm pretty confident that the actor itself is not the problem. It's the Task not getting released in time, I think.