Hmmm - well, there isn't a transmission latency per se -- that implies (at least to me) some delay between the switch wanting to send an UPB command and actually sending it. UPB controls that are ready to send a command do so immediatly, assuming there is no other UPB traffic on the line (in which case they wait for the traffic to clear).
As for UPB speed, I had a house full (70+ switches) of X10 switches and in the last 6 months have switched over to UPB. I can assure you the speed difference is very noticible -- at least 4x faster than X10. With X10, you could often count to 2 seconds before a command that was being sent caused a switch to come on. With UPB, while there is a delay, I think it is less than 1/2 second and probably more like 1/3 second.
Where the UPB delay that people really gripe about comes from is that when you press a UPB switches "on" paddle, it waits for up to 3/4 of a second or so before sending out the UPB command (if any) and turning on the connected load. This is supposedly because the switch needs to see if the user is going to double tap (which could fire an entirely different action). It is annoying, though you do tend to get used to it (well, most folks). I beleive PCS/UPB implementers have heard enough from their clients on this that I'm hoping this will be removed in the future.
On the topic of 3 way, there are two likely reasons for using a dedicated slave vs a seperate switch: cost and time. THe slaves are a lot cheaper than another switch and that 3/4 second delay and then the 1/3 second UPB needs to send a command starts adding up to be annoying.
Granted, if you want a multi-button controller at both sides of the 3 way, you have to use 2 UPB switches, but otherwise.
I have both setups (slave switches -- 4, and 2 UPB switches acting in a 3-way arrangement -- 2). For where I used the 2 UPB switches as a 3 way, the delay doesn't matter (it's for turning basement lights out after leaving the basement). But for the other places (hallways and stairways), I would stick with a tradional slave switch.
UPB control has been flawless (especially compared to my X10 system, which I thought had been pretty solid, but in comparison looks poor) and the speed of control has allowed me to use motion sensors, hooked to my controller to trigger UPB lights and links/scenes very effectively. While everyones experience is, of course, different, I've not seen any problems in high traffic areas with UPB switches responding to use or being controlled remotely (via other switches, sensors, etc). I'd agree that in a high traffic 3-way setting, you'd be best with a standard UPB slave 3-way setup.
Gerry