Hi Mark, thanks for that.
Between smoking a turkey and having dinner (hey it's Canadian Thanksgiving this weekend), I got some more time to work on this too. And I have some answers.
It's not a LR issue - and I was able to develop a test case using just a TIF file and Mac's Preview app to replicate it easily....so that took LR out of the issue.
I got some feedback from HP that they were able to replicate it on their Mac's - but they also found that the problem didn't show up on Windows (you remember you mentioned that your XP machine seemed to spool faster). That was a key piece of information because it indicated that it wasn't necessarily a problem with the printer's network implementation.
I kind of suspected this and worked at setting up the printer connection differently. Rather than using Apple's Bonjour, I set it up to use the printer's IP address directly (second tab in the Add a New Printer dialog box). I know my printer is on 192.168.1.199. And it uses Jet Direct.
Massive improvement, 10-15 times faster....now showing speeds occasionally peaking about 12MB/sec in the activity monitor. The practical implication is that I file I printed earlier today (48x43") took about half-an-hour to spool and tonight it's spooling in maybe a minute or less...once LR finishes rendering it.
* I didn't print all those
So, it's simple to lock down the IP address in my router so the printer sticks on "199" and I can live with that for a while and I'll feed this back to HP in the morning. They may have a different recommendation for how to approach it, but I suspect that Bonjour is the issue.