For a while now I've suspected that memory management on OS X is actually terrible. I have no really proof, because I just run everything memory intensive on Linux. At work I have a few Python script that eats a few gigabytes of memory, it reads in data, and basically just loops over it, selects the stuff it needs an generate a few XML file (10 to 20MB worth of XML).
All this takes maybe five minutes on Linux, on Mac OS X it will freeze the OS for long periods of time and take 40 minutes to run. In terms of hardware, the Linux box has less RAM, CPU and slower discs.
Regarding the Javascript stuff: Part of it may be that Chrome is the new Internet Explorer. People develop on and test for Chrome (because everyone uses Chrome, right?). Chrome does have the better Javascript engine, still, I believe that much of the slow Javascript on Firefox (an other browsers) could be avoided if people bothered testing a little more.