In the next beta, you should be able to do chapter splitting in the Code View, by marking where you want the chapter breaks and then perform a "Split on SGF chapter breaks" operation.
So if you have a very large file, you switch to Code View, add
all the break markers and
then split it. That should cut down the time it takes to split things.
The halt-on-save... do you have any images with spaces in the filenames? That causes the current beta to freeze on save. It's already fixed.
The long load time for large HTML files... that's still QtWebKit. I can't do anything about it.
But!
There have been some
recent developments in QtWeBkit regarding the rendering speed on Windows, and in QtWebkit 2.0 (scheduled for May 2010), it should become an order of magnitude faster.
At least, that's what the Nokia devs tell me. You can see the discussion in the linked issue report.
Let's hope they actually stick by their word this time.