10ms minimum to go to the next rack. For 99% of the requests. For the unlucky 1% you'll get outlier performance due to.... well... you spend the next sprint figuring it out and the next quarter trying to solve that scaling bottleneck.... It could be network, it could be some weird design thing, it could be redis, who knows? But yeah... enjoy figuring it out instead of focusing on your product.
1% of all requests is actually a lot of requests. And if you think 1% of your auth requests aren't 10x the median... you aren't measuring it hard enough.
The signals going through the wire and back are 2000ns in a big datacenter. There are orders of magnitude more delays in the processing of those bits than their transmission through space via twisted pair or fiber.
It's not 10ms to go to the next rack though unless your network is broken. You're much closer to 100us (100,000ns).
This is a very common pattern in networking, people underestimate the speed of light and overestimate the speed of network equipment.
Computers are not good at network requests because physics says they can't be - it takes orders of magnitude more time for light/electricity to traverse even one side of a datacenter to another than it does to go from the CPU side of the motherboard to the RAM side.
For plenty of use cases, the additional network roundtrip is very affordable. But it's simple math that explains why this might easily become half or more than half of the total load on your systems if you want to "just do an additional (network-mediated) hash table lookup" on every request.