While the time needed to actually send the file from a server depends
directly upon the file size, the 'startup' time once that data is
received by your browser can vary greatly depending upon what kind of
processing your document performs.
Note that, once the document HAS been loaded, there is NO additional
overhead, since the entire 'site' has already been read into your
browser.
In practical terms, depending upon your bandwidth, a file containing a
several megabytes, with hundreds of tiddlers is completely workable.
I've seen a few documents that even had over a thousand tiddlers, and
once they load, they seem to work just fine (though saving can be a
bit slower).
enjoy,
-e
Eric Shulman
TiddlyTools / ELS Design Studios