DispatchSourceTimer schedule (GCD swift macOS) stops after 40 seconds when the app is hidden

Question

For a macOS app (SwiftUI).

I'm setting up a simple timer using DispatchSource.makeTimerSource as shown below. It runs properly, but when the app is in background (hidden, minimized or not active), after 40 seconds it slows down a lot. Events are still fired, but every tens of seconds instead of 0.5 seconds. Any idea why? And how to make something keep running in the background at a normal interval?

@main
class AppDelegate: NSObject, NSApplicationDelegate {
    var bgTimer: DispatchSourceTimer?
    var count = 0.0

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        […]
        let queue = DispatchQueue.global(qos: .background)
        bgTimer = DispatchSource.makeTimerSource(flags: [], queue: queue)
        bgTimer?.schedule(deadline: DispatchTime.now(), repeating: .milliseconds(500))
        bgTimer?.setEventHandler(handler: {
            self.count += 0.5
            print("completed task from background ", self.count)
        })
        bgTimer?.resume()
    }
}

Thank you in advance :)

[Edit] Threads are also throttled after a little while:

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        let thread = Thread {
            while true {
                print("something")
                sleep(1)
            }
        }
        thread.start()
    }