Safari 3.1 just came out and with it, the awesome Develop menu, also for Tiger (my main dev. platform). I do profile my sites occasionally using Firebug, but the little recommendations that Safari's Network Timeline gives were surprisingly accurate. More specifically the little “You could save bandwidth by having your web server compress this transfer…” popup.
Indeed, Apache 2.x now comes with mod_deflate built in (instead of mod_gzip) and enabling it was a simple matter of:
AddOutputFilterByType DEFLATE text/html text/plain text/xml
SetOutputFilter DEFLATE
There is a lot more that you can do with it, but even those two simple lines cut my page size from ~500k to ~190k! Cool.
Unfortunately I'm not sure if that's on by default on Leopard, as all my production servers are very heavily “augmented” by now. :-) Worth a check.