in5 has no page limit on the document length that it can export (other than the speed of your computer), but longer documents require additional considerations.
For the sake of this discussion, a long document is one with > 60 pages.
Longer documents take longer to export, and they take longer to load in the browser. This article includes some tips to help with both.
Improving export time
Here are some tips to optimize export time with long documents.
Style your text
The fastest way to render a document is by rendering Text as Images, however, if you want to render live, selectable text there is one thing you can do to increase export time:
Style your text cleanly with Paragraph and Character Styles (and no overrides).
When text has has overrides, in5 creates new styles to describe the visual properties of the text. To create the cleanest, lightest-weight output, in5 compares the styling of the text to all the properties of the existing styles to see if an existing style can be used. This can be time consuming if you have a lot of un-styled text in your document.
Creating your own styles also ensures that your styling will be best preserved in the output. With un-styled text, in5 has to guess at which character in the paragraph best represents the underlying style and not a custom character style (e.g., a drop-cap in the first character, or bolded/emphasized text somewhere in the paragraph).
Rendering Text as Images is the faster export option (though not necessarily the fastest loading option in the browser).
Image Optimization
Pro tip: You can always turn off Image Optimization while testing (for a quicker export), and only turn it on for your final export to get the smallest files.
Exporting a custom page range
If you're only testing certain pages while creating your document, you can choose to export a small range of pages (or a single page).
To do so, choose Custom from the Export Range in the Basic section of the in5 export dialog.
Improving loading time in the browser
One of the challenges with long documents is that there's a lot more information and media for the browser to load. This can take time.
Here are some tips to improve load time.
Delay loading of page assets until they're viewed
By default, the Lazy Load Images option is selected when you export with in5 (found in the Advanced section of the export dialog).
Lazy Loading only loads images from the current pages, and the next page, but it does pull in all of the HTML.
Normally, the HTML text itself is not a lot to load, but this changes in long documents.
To get the best loading performance, choose the Multi-page Web Format when you export.
Instead of creating a single HTML file, this option will create an HTML file for each page of your document.
When your viewer reads your document in the browser, they will only have to load the HTML and media from the current page they are viewing.
This will dramatically speed up the loading time of the initial page.
Optimize Images
in5 has ability to optimize images and shrink the file size of the images in your export beyond even what Photoshop can do.
Since images are a big factor in file size, using this optimization will speed up the loading of your pages.
As of v3.6, there are now two types of Image Optimization: Standard and Enhanced.
Standard does a quick optimization pass on all SVG, PNG, and JPG images created by InDesign during the export process. This setting is available on Pro licenses and higher. It can reduce the size of your output significantly and speed up loading time in the browser.
Enhanced does a slower, more thorough shrinking of the files during export, which results in even smaller files. It also goes through and consolidates any duplicate images created by InDesign during export, saving even more file size. This setting is available on Gold licenses or higher.
Note that optimization can slow the export process somewhat (described above)—though it's must faster now in v3.6 —so you may want to save it for your final export.
Export fewer images with grouping
To get the last few bytes squeezed from your document, you can group images that don't have interactive content and choose Render Groups As Images (in the Advanced section of the in5 export dialog).
Export fewer images with reuse
If you find yourself using images on multiple pages, consider moving those images to a master page. In most cases, that will cause in5 to only export the image once and reuse it.
You can also Source Image option to reuse web-friendly images (jpg/png/gif) that you've placed in your InDesign document. More on Image Quality settings.
Upgrade your web hosting
If you've done everything to optimize your document, the last limiting factor may be the speed of your web hosting.
Upgrading to a faster plan is an easy way to speed up the loading of all of your content.