Hello again, all. I found some intereting results.
Late yesterday I fired up my old G4 Tower (1.25Ghz SP) running 10.4.11, Safari 4.1.3, Camino 2.1.2, FireFox 3.6 (?), and TenFourFox (most recent).
First I tested its internet connection speed and confirmed it at 13-15MBps from two different tests pinging two different servers each. The I accessed my Yahoo mail from each browser.
Safari, Camino, and FireFox took minutes to load the Yahoo home page and get to the Mail page. The status line showed it was waiting for other sites, some ads. The most common site not part of yahoo.com these pages were struggling to hit was yimg.com---I assume some yahoo image site.
The bottom line was that all three browsers were nearly unusable with myYahoo.com or Yahoo mail from a PowerPC Mac.
Then I tried the same site wth TenFour Fox and the same pages loaded almost as fast as on my 2.93gHz quadcore i7 iMac.
TenFourFox looks like it's dealing with some PPC-unfriendly code on Yahoo sites much better than are the "traditional" browsers.