Dispatch queues

If i set the timeInterval to less than 0.1 , UI starts blocking, combo box does not update,
can not select items in combo box, etc.
This is just to get to know the macos environment better. Like to experiment things. I like to make it real fast. How can i do that?.

this runs in swift cocoa project.

aTimer = Timer.scheduledTimer(timeInterval: 0.1, target: self, selector: 
#selector(afunction), userInfo: nil, repeats: true)
@objc func afunction() {

        DispatchQueue.global(qos:.background).async {

		*here -- call some C function to do background processing and return*

  		//update UI
                DispatchQueue.main.async { 

                if let n = N {

                    for i in 0 ..< n.pointee { // n elements

                            self.cbbox.addItem(//insert ...)
                            self.cbbox.selectItem(at: 0)

Rather than using a periodic timer, try using a one-shot timer and restart it from inside the afunction after updating the UI. The periodic timer firing at that rate is probably causing severe congestion.