Google has just recently announced landing page speed will be used to calculate quality score. As to how weighted it will be I'm not sure. My guess is that it will not be a heavily weighted factor. If your Joomla page is slow loading, then yes it would take a bit of a hit, but again, my personal opinion is that it won't be very much.
Emm.. If like fro example my landing page is super loading using HTML and css but as I've placed flash its a bit slower than the other, will that effect anything? Thanks p/s- sorry to bump into your thread OP.Cheers