Exactly the same experiences here!
FB works just fine in all other browsers. But in safari, all pics, info 'windowlets' when you mouse-over the upddates in the right column, etc etc don't show, or load soooooo sloooooooow. Lot of blank spaces on the page, only. And this stuck window even slows down or blocks all traffic in the other safari windows as well. In Safari5, I could go on browsing in the other open safari windows w/o problem, even when one window got occupied with a slow download....
But: I have managed to narrow down the problem to the following:
Are you running Facebook on a secure connection? Https ? This seems to be the culprit, for me, at least!
I turned off 'secure connection' temporarily on my Facebook page. At once, the Facebook pages started to work as normal, and really fast. But when I turned it back on, it immediately goes back to barely working.
So I guess it is how Safari6 handles traffic through SSL/TLS. Especially on a page like Facebook, that is updated continously, receiving a full load of new info every second, new pictures,columns, ads etc.
I have also done some of the traditional exercises, just for the sake of it:
turned off and on again Java and Javascripts, and rebooted safari, deleted and reinstalled java, also cleaned safari a number of times, even manually deleted all safari caches in Library. And also went through a full OS cleanup with Onyx... No improvement.
If it is https:-related for you too, and you don't like the idea of an unsecure FB connection, the workaround until Apple can sort this out, is to either use another viewer like Chrome, or just invoke another viewer temporarily from within Safari. (In the Developer drop-menu). Safari runs like a charm, except from FB, so I use the latter solution myself.
Question: did you do an upgrade /'patching' from lion on both macs? Or did e.g. the imac get a clean install, and therefore works better?
morten
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Tech stuff:
Safari build is: 6.0 (8536.25) - Imac mid11, 27", 12 gig, I5 3.1 Ghz; AMD Radeon HD 6970M