I am a sysadmin for an online retail catalog co. we tried running one of our catalogs on 100% SSL and we just couldn't sustain it.
customers reported poor performance, we had to stop using akamai (we have > 100,000 images), performance was visibly worse from every measurement.
We tried for about a month to get it worked out, but in the end we gave up. customers with old hardware could hardly use the site at all and got frustrated.
forget about flash downloads and background music and sound samples, it was just too slow.
maybe someday we will do another test, but for now we just accept it and try to do the right things on other fronts.