[KevinH]

If the slowdown is due to the Preview panel being open, then you are trying to load a much much too large xhtml file.

It is impossible for a browser to render just part of an html file. It literally has to grok the entire file to determine the proper layout and sizing of all of the elements. If you try and keep the entire book as one huge xhtml file, the browser literally has to re-layout the *entire* book for each and every small change.

Using Sigil's split-at-marker (after using find and replace to insert them properly) you should split your large xhtml file into separate files, one for each chapter. That is the recommended format for epubs which have to work on large range of devices, browsers, and e-readers.

This also speeds up Sigil immensely as having to relayout an entire books's worth of xhtml for every minor change is just plain silly.

Whoever recommends people to use one large xhtml file is really off the mark and does not understand anything about how technology or browsers really work.

Even Kindle's split the text internally into sections of about 6000 chars or under and tries to show each section independently (as did the old mobi format as well). This was done for speed of loading and compatibility across devices.

I have no idea how the newer KFX format handles a large single file but my guess is they are splitting and rendering it in sections or even pages dynamically and not trying to render the entire book at once.