> I often use cellular internet with a marginal signal that is oversubscribed, and 5 second ping times are quite common. Even 20 seconds at some times.
> In other words, it may not be a bufferbloat problem
Whenever you experience ping times over 1 second, something somewhere is buffering your ping (or pong) packet for more than 1 second. Even if this buffer is not "bloated" strictly speaking, holding on any packet for that long is WRONG and is definitely a BUG.
As an example, any link retransmission technique with timeouts over 1 second is simply not compatible with TCP/IP. Making that link technology compatible with TCP/IP is as simple as making it timeout (and drop packets) much, much sooner. It is really that simple.
Now again, finding the *optimal* timeout value is a very difficult problem. However, reducing a multi-seconds timeout to a reasonable 100-150 milliseconds value is NOT difficult at all and will make every user happier, while upsetting none.
Here is a supermarket analogy (for a change). You know that queues always become too long at peak time. Customers complain about it. You have money for two extra tills. But you do not proceed because it is oooh so hard to find the optimal number of tills.
> In this case the way forward looks like newer cellular technology.
This is throwing out the baby with the bath water. And the newer technology might make the same mistake again. And in any case it will not displace 2.5G/3G everywhere overnight.
> Even so, it's not clear that "speed of light" is an achievable latency
Of course it's not; you need some reasonable amount of buffering for a number of reasons.