@tibert: Sadly it is difficult to verify if third party chargers are compatible with Fast Charging on the Tab S5e. All I can suggest is that you use the charger plug and cable that came packaged with the device, to ensure that you don't encounter any issues.
Whilst some beneficial advice has been provided as @AndrewL has indicated there may be issues with 3rd party chargers and Samsung only recommend using their own chargers.
However anyone who decides to use another supplier should at least ensure they are a trusted source. See more here:
@Lastboyscout I am aware that some customers have been fine with third party chargers, however just saying it can not be guaranteed. Anyway just reiterating that it is important to do some research, thank you for your input.
thanks for your answer, I looked on my charger, it has only 5v and 2 or 6 A, it would explain why it cannot do fast charging.
But then why would it slow down the charging at a so low rate when using the OnePlus cable, and use a normal charging speed when using another third party cable, but with the same charger ?
As @CrazyTom2k2 and @Lastboyscout say there is a safety mechanism built in and the speed will reduce once a significant charge is reached. The battery is under the most strain once it reaches the 80% level and in fact many analysts would suggest that not charging to 100% will maximise the battery life. Some tips and and debunked myths in this article. https://charbycharge.com/how-often-should-i-charge-my-phone-to-prolong-the-battery-life/