Remember that LoRaWAN packets have a fairly high header overhead, so by the time you get to SF12 you are spending around 3/4 of a second just to encode the headers and checksum, before even trying to move a single byte of useful message data payload.
However, this also means that the issue may be moot in some places - for example SF11BW125 and SF12BW125 are not usable with US915 because the overhead would consume the entire permitted packet duration, with no data moved. In Europe the restriction has more to do with accumulated airtime, so slow packets moving actual data are possible - though still extremely expensive of opportunity.