#!/usr/bin/env python

import sys
import time
sys.path.insert(0, '/usr/lib/python2.7/dist-packages/')
from qgis.core import *

composedURI = '/home/martin/smetiste/test-sqlite/p.sqlite|layername=p'
layer = QgsVectorLayer(composedURI, 'p', 'ogr')
features = layer.getFeatures()
fieldID = layer.fieldNameIndex('value')

start = time.time()
layer.startEditing()
        
layer.updateFields()

for feature in features:
    featureID = feature.id()
    layer.changeAttributeValue(featureID, fieldID, -1)
        
layer.commitChanges()

print ("Time elapsed: {}".format(time.time() - start))