The code editing component of ShiftEdit is by far the most important. The current version offers a choice between MDK editor and CodeMirror. They both have their pros and cons and are maintained by individual developers. Both struggle with large files and because of their implementation it's hard to see that ever being fully resolved.
The Bespin project by Mozilla is also providing a stand-alone editor component. It has a more robust implementation and greater resources behind it. It is integrated into the development version of ShiftEdit. Currently the Bespin component has a few bugs and is lacking some features. Once these are resolved and Bespin is fully integrated, ShiftEdit should be ready for prime-time. Then I'll be able to eat my own dog food and end my reliance on Dreamweaver.