I submitted online 5 days before my due date and was rejected. I was not given a reason but asked to go immigration. I managed to get an online appointment and when I asked the officer why my application which was correctly filled was rejected she said I need to submit the it 14 days from the due date! Not any later. This was not my understanding so I was kind confused and none the wiser.
Yes it used to be rather swift. Mine was approved the day before it was due. I heard of the same with other people. My plan was to go on the due date if the online approval didn’t come on time. So it really depends on how comfortable you are with the suspense:) and if you can make the date. Good luck!