Fetching data using firebase but UI freezes on simulator - ios

Question

I am new to using firebase, I am trying to fetch data from firebase.
I get the latitude and longitudes of driver from firebase and append them in latitude array and longitude array and then create the polyline on google map.
When I run the code on simulator UI freezes, zoom in and zoom out takes time, but on real device, iPhone 4s, map does not open.
My database Structure:
func API()
{
let rootReference = FIRDatabase.database().reference()
let DriverId = String(Driver_id)
print("DriverId = \(DriverId)")
let currentDatePath = rootReference.child("gogoapp_driver/\(DriverId)/\(dateFormatForDriverLocation!)")
currentDatePath.observeEventType(.Value, withBlock: { snapshot in
for item in snapshot.children {
let child = item as! FIRDataSnapshot
let latitude = child.value!["latitude"] as! String
let latitudeValue = Double(latitude)
self.latitudeArray.append(latitudeValue!)
let longitude = child.value!["longitude"] as! String
let longitudeValue = Double(longitude)
self.longitudeArray.append(longitudeValue!)
}
self.createPolyline()
})
}
func createPolyline()
{
if latitudeArray.count > 0
{
let lastLat = latitudeArray.last
let lastLong = longitudeArray.last
let coordinatesForCamera : CLLocation = CLLocation(latitude: lastLat!, longitude: lastLong!)
let camera = GMSCameraPosition.cameraWithLatitude(lastLat!, longitude: lastLong!, zoom: 12)
self.googleMapsView.camera = camera
let path = GMSMutablePath()
var line = GMSPolyline()
for( var i = 0; i < latitudeArray.count; i++)
{
let currentLocation : CLLocation = CLLocation(latitude: latitudeArray[i], longitude: longitudeArray[i])
let longitude :CLLocationDegrees = longitudeArray[i]
let latitude :CLLocationDegrees = latitudeArray[i]
let c1 = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
path.addCoordinate(c1)
line = GMSPolyline(path: path)
line.strokeWidth = 5.0
line.map = googleMapsView
marker.position = c1
marker.map = googleMapsView
marker.title = "\(ChildName)"
}
line = GMSPolyline(path: path)
line.strokeWidth = 5.0
line.map = googleMapsView
}
}