How to fix “Unfortunately Contacts Has Stopped” or Very slow contact loading
This is very likely to be caused by a bug in an update, but check with other people who have the same phone to be sure. Assuming you want to keep your system updated, I suggest you move to another contacts app like Contacts+, which offer more options than the stock contacts app.
You might use CleanMaster to see which apps are causing the phone to go slow.
Meanwhile, I would suggest contacting Huawei (if you haven't done so already) so they will fix this issue in a later update.