In general the batteries charge at a consistent percentage rate, not kWh/mph rate. The smaller the battery, the lower the kWh average rate, but in theory, all batteries SHOULD charge 10% to 80% in the same time. This is because each cell can only take a certain current when it is at a certain charge level and temperature, and the bigger the battery, the more cells you have.
You'll find in practice that it's hard to achieve Tesla's advertised 40 minutes from 10% to 80%. My experience has been on average you are likely looking at closer to 50 minutes, although some superchargers are definitely better than others, either consistently or randomly. Pairing does matter, but not as much as you may think, as any vehicle you are pair with is likely tapering. If someone with an empty battery has just plugged in, you want to avoid pairing with them, and always try to find an unpaired charger when possible. If you can check the rate of green flashing on the charge ports of cars you may be paired with, you can get an idea of how much current will be available to you. You'll want to pair with a car with a slowly pulsing green light, meaning it's almost fully charged and drawing less current.
Another unavoidable variable is the supercharger itself. Sometimes they are just not working well. There has been much discussion on these boards about what variables contribute to poor supercharger performance...there's a nice long thread on the california forum I know about/participated in called something like "California superchargers not providing optimal rates" or something like that.
In the end, if you are buying an EV, you cannot expect to be refueling anywhere near as fast as an ICE car. However, superchargers are a really valuable and powerful infrastructure system, that will usually be pretty amazing. You must, however, be prepared to be very frustrated at some point in your Tesla ownership, whether it's a long line, rude tesla owners who want to charge to 100% with a long line, superchargers experiencing technical issues, or chronically slow superchargers like Burbank. Things will probably improve in the future but that has been my experience so far. I recently lost my tesla to an accident, and ordered a new one, so these problems obviously didn't deter me.