Hi
I found this on another forum, which sounds like a good idea to me, and has worked for those who have tried it seems, even though I have to say I haven't tried it yet:
http://forums.imore.com/iphone-6s/375329-6s-phone-contacts-app-freezing.html#pos t2832667
"Sometimes, but not always, when there is an update and old system files are overwritten with new system files, file corruption can occur. When this happens you get odd behavior with some apps, or they just crash upon opening and won't run at all. The only way to fix corrupted files is with a clean install of the operating system.
Make a backup using iTunes (remember to use the Encrypted backup feature if you want your health data, wi-fi passwords etc, backed up), and then factory reset the phone. Once you've completed the factory reset and gone through the initial setup of the phone then you can use iTunes to restore your phone (ie; it'll seem like you never reset the phone even though you did)."
- By 'iMore Specialist' - Sherry_B