Dave over at Nisus wrote me to let me know that the next release of Nisus Writer Express 2.0 will be considerably faster than the current version when working with long documents -- this could be the end of Microsoft Word on my iBook! Dave let me know:
[T]he next version of Express should be sped up fairly dramatically. The build I have seen was faster by not a little ... it is faster. However, it is also quite crash prone at the moment. We should have a release in mid to late October, with some luck.That is great news! I cannot wait for the release. Thanks for the update Dave!