This makes sense as to why people get quicker decisions from in-person appointment and not via IDV App. Although UKVCAS maybe sending data to UKVI within 24 hours, but what happens with data is different:A caseworker needs Biometrics in your current case file to proceed. Some caseworker will pre-assess the application while some won't.
If you have been chosen for biometric reuse, then after you submit your facial image through IDV App; your biometrics from old case file are put in a queue to be injected in your new case file. This queue is 3-4 weeks long.
When you attend an in-person UKVCAS appointment, your biometrics are immediately available to your current case file.
By doing in-person appointment, you are saving yourself time it takes to move biometrics from old file to new one - which can be around 3-4 weeks.
However whether you will get quick decision or not depends on whether your application was pre-assessed prior to biometric submission.
In-Person Data - straight to current case file
IDV App Data - put old biometrics in queue for injection into current case file