as I neared the remaining %, it slowed down
Depends on the Model, and latest freebie-improvement-giveaway software update
, but in general terms 10% - 70% will charge "flat out", somewhat slower to 80% and then increasingly dramatically slower above that.
Arriving with cold battery, or below 10%, will mean that initially the charging is very "gentle" and, if appropriate, significant power being used to warm the battery.
Setting SatNav to location of the Supercharger (or it routing you "via" one) will cause the car to pre-condition the battery as you approach the Supercharger so that you charge at optimal speed as soon as you plug in (seems to be a bug that if you use the APP to calculate the route and then" transfer" that to the car then this pre-conditioning doesn't happen, so reselect on SatNav to force that if you know the battery is cold and you don't have a significant drive to get there (which will, of itself, warm the battery)
As already said avoid "pairing" at A/B stalls with an car already charging.
If you then find that charging doesn't ramp up to the kW you normally get the move to another stall. Sometimes one stall can be operating at lower power (would be nice if SatNav told you any stall NOT to use ...)
I recommend watching a charge and seeing what the kW is at various percentage state-of-charge so that you get a feel for what it should be.