Malcolm,
This is an Editor problem. Or, rather, it's a memory allocation problem. I used to use Homesite a *long* time ago and I do recall it was better at handling large files.
There are a number of issues tied up with this... I don't have a profiler installed but I'm willing to bet that Editor allocates a block of memory (RAM) when it loads the file. Inserting a character means moving everything past that point forward one space in memory... and then redrawing the screen (and I'm guessing predrawing everything past). Add to that the overhead of the code highlighter and you can start to see where it might get a bit challenging