How can I gain the speed on facial recognition

edit

I am using my Raspberry to perform LBPH facial recognition. But it takes a long time to load the "trainer.yml" and It got stuck when I tried to draw a rectangle around the recognized face.

#coding=utf8
import cv2
import os
import time
recognizer = cv2.face.LBPHFaceRecognizer_create()
recognizer.read('face_trainer/trainer.yml')
cascadePath = "haarcascade_frontalface_alt.xml"
faceCascade = cv2.CascadeClassifier(cascadePath)

font = cv2.FONT_HERSHEY_SIMPLEX

alarm = 0
idnum = 0

names = ['1', '2']

cam = cv2.VideoCapture(0)

minW = 0.1*cam.get(3)

minH = 0.1*cam.get(4)

while True:

    ret, img = cam.read()
    gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

    faces = faceCascade.detectMultiScale(
        gray,
        scaleFactor=1.2,
        minNeighbors=5,
        minSize=(int(minW), int(minH))
    )

    for (x, y, w, h) in faces:
        cv2.rectangle(img, (x, y), (x+w, y+h), (0, 255, 0), 2)
        idnum, confidence = recognizer.predict(gray[y:y+h, x:x+w])

        if confidence < 85:
            idnum = names[idnum]
            confidence = "{0}%".format(round(100 - confidence))
        else:
            idnum = "unknown"
            time.sleep(5)
            alarm = 1
            break
            confidence = "{0}%".format(round(100 - confidence))

        cv2.putText(img, str(idnum), (x+5, y-5), font, 1, (0, 0, 255), 1)
        cv2.putText(img, str(confidence), (x+5, y+h-5), font, 1, (0, 0, 0), 1)

    cv2.imshow('camera', img)

    k = cv2.waitKey(10)
    if k == 27 or alarm == 1:
        cv2.imwrite('face.jpg',img)
        break

#os.system("python /home/pi/mail.py")
#os.system("rm face.jpg")

cam.release()
cv2.destroyAllWindows()
os.system("python /home/pi/python2.7_guest/main.py")