Camera feed slows down using AVCaptureSession on iOS. How can I speed it up?

Now I am trying to allow users to take pictures in my application without using UIImagePickerController. I use AVCaptureSession and all related classes to load the camera channel as a sublevel in full screen, which I have on one of my controllers. The code works, but, unfortunately, the camera loads very slowly. Usually takes 2-3 seconds. Here is my code:

session = [[AVCaptureSession alloc] init];
session.sessionPreset = AVCaptureSessionPresetMedium;

if ([session canSetSessionPreset:AVCaptureSessionPresetHigh])
    //Check size based configs are supported before setting them
    [session setSessionPreset:AVCaptureSessionPresetHigh];

[session setSessionPreset:AVCaptureSessionPreset1280x720];

CALayer *viewLayer = self.liveCameraFeed.layer;
//NSLog(@"viewLayer = %@", viewLayer);

AVCaptureVideoPreviewLayer *captureVideoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:session];

captureVideoPreviewLayer.frame = viewLayer.bounds;
[viewLayer addSublayer:captureVideoPreviewLayer];

AVCaptureDevice *device;

if(isFront)
{
    device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
}
else
{
    device = [self frontCamera];
}

AVCaptureDevice *audioDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeAudio];
AVCaptureDeviceInput * audioInput = [AVCaptureDeviceInput deviceInputWithDevice:audioDevice error:nil];
[session addInput:audioInput];

NSError *error = nil;
input = [AVCaptureDeviceInput deviceInputWithDevice:device error:&error];
if (!input) {
    // Handle the error appropriately.
    //NSLog(@"ERROR: trying to open camera: %@", error);
}

[session addInput:input];
[session startRunning];

stillImageOutput = [[AVCaptureStillImageOutput alloc] init];
NSDictionary *outputSettings = [[NSDictionary alloc] initWithObjectsAndKeys: AVVideoCodecJPEG, AVVideoCodecKey, nil];
[stillImageOutput setOutputSettings:outputSettings];
[session addOutput:stillImageOutput];

Is there any way to speed it up? I already tried loading it into another thread using Grand Central Dispatch and NSThread, and although this stopped the application from freezing, it made loading the camera even longer. Any help is appreciated.