Does this only happen with older superchargers?
Lots of good answers in this thread, but nobody has specifically answered this one. Interestingly, this happens
more at V3 stations. Let me explain...
We all know V3 Superchargers can deliver 250kW, right? What if the station is full and every car is ready to receive 250kW, can the station deliver that much power? Nope!
V3 Superchargers use 1 "cabinet" (the big white box thing) for every 4 charging connectors. Each cabinet can do ~375 kW or so, or ~94KW per connector.
Note: the cabinet can route power however is needed between cars, so it doesn't matter where you plug in. The station can do this
between cabinets as well (more or less) so as a driver you still never need to worry about where to plug in at a V3-only Supercharger.
So if we do the math at an 8 stall Supercharger (typical, but small) it only takes three cars requesting 250kW at once to fully max out the station, or 5 cars requesting 150 kW. Not a lot! At the point the available power is (based on some rules) balanced across the cars and likely nobody will get full speed.
That sounds terrible! Actually, it's not as bad in practice due to the rather short time 3 and Y can charge fast (only a few minutes and then they taper) but I have noticed if you arrive at a busy 8 stall V2 station ready to accept 250kw it's common to "only" get low 200s.