[KevinH]

There are no hard coded limits in Sigil since we worked around Chrome's 2 gig limitation per chapter. There are of course memory and processor limitations depending on your machine, operating system, etc.

That said, Sigil's editor is line based so having super large files without line end characters will make it crawl. Use of Preview which reloads the chapter at set intervals after each change will make it crawl as well with huge files. Enabling automatic spell check of huge files also slows things down.

So adding proper line ends, properly splitting contents into reasonable sized files, turning off auto spellcheck, turning off preview (or make it non-visible until it is needed), all help. As will using a computer with lots of ram and processing power, that has an OS whose security software does not cause memory leaks of pages used by jit compilers for javascript (ie. not Windows 10 and earlier without Microsoft's recent fix) etc will help as well.

The key to Sigil is to remember what the program is designed to do. It is meant to be a tool to help epub devs create reflowable epubs meant to be distributed to actual reading devices.

That means epub devs should:

1, keep the size of each chapter file reasonable (the entire book in one chapter is very poor design that will break many ereaders or make page turning horribly slow)

2. should not try to create monster book collections in one epub for the same reasons

3. should produce reflowable not fixed format layouts, use pdf for fixed format layouts

4. should minimize image file sizes whenever possible

5. should minimize additional fonts to those actually needed, etc

6. should understand and properly use shared css files

7. should design their content to be Accessible

etc.

Keeping all of that in mind, if you are not working with single huge files of content, and are using an up-to-date computer with lots of ram (32 gig or more) and are seeing slowdowns after long use (and after understanding the above) please provide a testcase epub, and report your Sigil version, platform and os version. Also try disabling anti-virus and related software for Sigil, look for and remove any broken system extensions or system utilities that interfere with Sigil and Qt, etc.

Once I have a testcase and can reproduce your slowdown, I will look to see what if anything can be done to speed things up or avoid the issue.