It is very quick and has zero problarms. Please do not spread untrue rumors.
First of all, stating that something is perfect and has zero problems in IT confirms that your origins are not from IT, and you are not using this feature heavily. There are moments that I need to access my iPhone several times in just few minutes - that means for example 10 times in 2 minutes. And waiting 2 seconds just to access phone to have TouchID recognise finger is slow for me. Of course - you're right that it is fast in general considering nowadays technology, but it is not fast enough. Accessing iPhone without TouchID takes zero seconds delay, accessing with TouchID is 2 seconds delay, 2 seconds multiplied by 10 is 20 seconds delay, not just zero or minimal. Possibility to access phone without passphrase/TouchID within 15 minutes is total zero delay, in opposite of total 20 seconds delay - is that clear?
And yes, TouchID has problems, especially outside during bad weather - snow or rain is problematic and makes overall use of TouchID frustrating.
Blocking possibility to temporarily switch off TouchID for 15 minutes after first use (as it is with passphrase) is simply speaking a serious UX issue.