Have you pulled your notification bar down by sliding your finger from the top of the screen down. When you open the notification bar and and scroll to the left you will see the sync button, click on it to turn off sync. This will stop syncing on any of your programs that you are syncing. Repeat process to turn sync back on, this should be the easiest way to turn on and off sync. You can also try by going to settings, then accounts and click on the email you are using and uncheck the sync box. If you don't see your email account click on add accounts and find your email there and add it to the list and click on the three buttons at the top right hand corner and uncheck sync and that should stop the syncing on that email account. If it is gmail that you are using go settings, accounts and then gmail and go to the three dots on the upper right hand corner and click on cancel sync or click on gmail.com, there and you will see a list of what is being synced. Go to contacts and deselect the button there. You can repeat the processes to turn sync back on. If you have a slow wifi connection it will take a long time to sync that many contacts. Turning off sync to your email accounts will require you to sync your email manually in your account.