Performance of IFRAMES and Cached js/css files

[PCTeacherman]

I am an experienced developer, but fairly new to JavaScript and Web Development. I have two questions on perormance I'm hoping someone here can help with.

1 - It is my understanding that using js files and css files is cool since they can be cached... but I've also been told that the client must still burn a round trip to confirm the cached version is up to date. Considering the small size of many js files and css files, is it really all that much faster to verify the cached version versus actually getting the file? Is it really burning this round trip (on subsequent calls) or is it just running with the cached version?

2 - I've been asked to help improve performance on an existing page. It is an ASP file, so it is parsed, generates an HTML file that is sent to the client. The generated HTML file includes several IFRAMES sourced to either .htm or .asp pages. It is my understanding that each of these is another GET, yes? So couldn't I improve performance here by using Server Side Includes? (Political reasons prohibit me from simply incorporating the info into one asp page.)

Thanks so much for your thoughts and advice!

[Bernard Marx]

must still burn a round trip to confirm the cached version is up to date

That really depends upon the user's browser settings concerning checking for new versions. IE, for instance, has 4 levels:
{Always, Every time IE is started, Automatic, Never}.

This suggests that a new version will only confirmed, during the course of a site visit, if option #1 is active. The default is, I think, "Automatic". I have no idea what "Automatic" means though.