If your TouchID is failing that often, there's something wrong. The only time TouchID ever fails for me is if there's something on my fingers (usually water) or if my fingers are extremely cold. Even when it fails, I can enter the passcode immediately.
I'd start by deleting your fingerprints and re-adding them and then testing. You might also consider backing up your phone and then restoring from your back up.
See this article as well:
Get help with Touch ID on your iPhone or iPad - Apple Support
Meanwhile, consider turning it off.