Aside from just load time, there's another point that you might want to consider: the last time that I checked, Google would only index our particular pages up to 1 MB (though this value certainly isn't static, and 5 MB might be the new limit in the near future / already for some sites). So, if you're not splitting this file up, you should also be aware that not all of the contents will be indexed by external search engines.
Since this particular finding aid of yours has already been split into 10 different series, though, I think that that would be most logical method to split things up (and that should take care of both problems, while presenting one more problem of making sure that everything is still presented in a user-friendly way).
From: Code for Libraries [mailto:[log in to unmask]] On Behalf Of Brian Tingle
Sent: Monday, December 06, 2010 4:15 PM
To: [log in to unmask]
Subject: Re: [CODE4LIB] HTML Load Time
> If I split the page into say three parts and then combined them on one page
> using the include function of PHP, would I still have to same problem?
I don't think this will really help any, because the files would still
be combined together on your side
of the server and the large file will go over the wire and be loaded
into a ~5MB file in the local browser.
The static HTML page is pretty much as fast as you can get on the server side.