As someone who has been giving advice on visas and processing times for over 10 years, the processing times we have experienced have always corresponded with the processing times published, based on receipt of documents in Sheffield.
However:
1. The processing times published on the website are for ALL applications - priority and non-priority combined
2. The processing times currently on the website are 3 MONTHS OUT OF DATE. They are for DECISIONS MADE in February 2017... and back in February, even non-priority applications were being decided within 30 working days. Right now though, they are taking around 60 working days from receipt in Sheffield.
3. The online application is not transmitted anywhere so they cannot actually start the processing until you mail the printed application... so it makes no sense to start the count from biometrics
4. What if you give biometrics but then decide not to apply after all - if the processing time was taken from biometrics date that would totally skew the data because there would be an indefinite processing time based on the applications that are never sent to Sheffield
5. And following on, what if for some reason it takes you a month to get the documents to Sheffield after biometrics? You would then have added 20 working days to your own processing time - and UKVI will not be held responsible for your tardiness