An electric car charging at high speed slows dramatically once the battery is most of the way full. This is a property of the cells rather than a limitation of the charger.
The charge curve is not flat
Charging speed is plotted as a curve across the state of charge. Peak power is available in the lower portion and tapers steadily as the battery fills.
Manufacturers quote a peak figure, but that number is only sustained across part of the range. The average delivered over a session is considerably lower.
This is why charging times are usually stated between two states of charge rather than as a time to full, since the final portion behaves so differently.
What is happening inside the cell
Charging moves lithium ions from one electrode and inserts them into the other. As the receiving electrode fills, there is progressively less room available.
Pushing current harder at that point risks lithium depositing on the electrode surface instead of inserting into it, which permanently reduces capacity.
The battery management system prevents this by lowering current as the cells approach full, trading time for the life of the pack.
Heat sets a second limit
High current generates heat throughout the pack, and cells have a narrow band in which they charge efficiently and safely.
Cooling systems remove that heat, but they have a finite capacity. Once they cannot keep pace, the car reduces power to stay within limits.
This is why a second rapid charge shortly after the first often runs slower, and why cold packs also charge slowly until they are warmed.
Why the final portion takes so long
The taper is steep enough that the last part of a charge can take as long as the entire lower portion did.
Charging networks are aware of this, and pricing by time rather than energy makes the slow tail expensive for the driver and unproductive for the operator.
The result is that long-distance routing increasingly assumes shorter, more frequent stops within the fast part of the curve.
Why quoted charging speeds vary so widely
A charger's rated output is a maximum, and the car requests whatever it can accept given temperature, state of charge and pack condition.
The lower of the two figures wins, so a powerful charger paired with a cautious vehicle delivers the vehicle's number.
Understanding which side is limiting explains most of the difference between advertised and observed charging times.