After numerous posts here with people complaining that their fast charging was indeed much slower than before, I decided to investigate a bit.
First thing I noticed was that my phone wasn't as warm as before after charge is complete (to 80% for me) and I unplug it. So I used Accubattery to check the charging values and played with the charger a bit.
It turns out that the charging speed is greatly throttled by the temperature, with a cut off at only 33°C (91°F), which is much more agressive than before. This should be good for battery life, but clearly the quick charge feature is now not working as advertised. Note here how the current drops after it reached the threshold: https://imgur.com/a/F7g1CAw from 3000mA to 1400mA.
The charging speed is greatly affected by this of course, and the fast charging will only resume if the temperature drops under 31°C (87°F). If your phone never cools down, it will be stuck with this low charging current, much lower than the battery and charger allow (even taking into account the current decreasing as battery get filled)
These are totally undocumented changes and I really do not like that. The charging now performs poorly in certain occasions and people might just think their battery/phone or charger are bust (one of the post mentionned the OP buying a new charger to no avail, thank you Samsung).