"I'm looking for general advice on how to prevent the main site from loading quickly"
That contradicts the title of your posting.
Optimize/resize all graphics that you import into the file before you import them. Otherise, load all sizeable content dynamically, especially sound and video files. Which types of images you use depends on the nature of the images and the intentions. Generally, between jpeg and gif you will get the best savings bite-wise. Anything you can draw using the Flash drawing tools is a bonus in the savings department.
Instead of using UILoaders, use code-based Loaders. Using Flash components tends to add weight when you don't need it.
Thank you! I meant to say prevent the site from loading too slowly, I can't find an option to edit my original post for some reason. But thank you for the advice! Would you say it's a bad idea to import images directly from Illustrator, or is it better to export them as jpegs or gifs? I'd been doing a combo of exporting as the web & devices optimized pngs and just directly importing them; the pngs seem to loose a lot of quality though.
If you have vector drawing in illustrator, then importing them as vectors into Flash is the better way to go. I do not have alot of experience working with Illustrator (in any regard, nevermind in association with Flash), but what I usually do with illustrator graphics is copy them in Illustrator and paste them into Flash instead of importing them. I don't know if this buys me anything, but what I end up with is just the graphic elements I need.
This may be a really, really stupid question, but is the size of the .swf exactly tied to the size of the .fla file? Eg., if there are items in the library of my flash file that aren't being used (for example, different versions of an image that I'm trying out) in the actual movie, does that affect the size of the .swf?
Also, more generally: is it always preferable to dynamically load external .swfs? I'm looking at a situation with needing to have maybe 20 different 'pages' of a site, each with a few basic buttons. Is it likely to be a better idea to keep all these in the main swf, or to load each one using a simple code-based loader as suggested above? I guess the actual question is what slows down a flash movie more, code or images?
Nah... really really stupid questions only get asked in non-Flash forums. The size of the swf is not tied tot the size of the fla. The fla has alot more baggage that inflates its size greatly compared to the swf it creates. The swf will end up bearing the weight of whatever content you put in it.
If you want to see what's adding weight to your swf, go into your Flash Publish Settings and select the option to Generate Size Report. When you publish the file it will display a detailed report of each element and the weight it contributes to the swf.
As far as dynamically loading swf's, it is often a necessity, unless the swf is strictly an animation. If you import an swf into an fla file, all of the actionscript that might have been in it is stripped out.
Code will have negligible weight in a file. Images (large/heavy ones) can put lead shoes on a file, along with video and sound files.