[KevinH]

Okay I have Preview being loaded asynchronously in my local tree which helps but the bulk of the time is spent in merge pre-validating xhtml is well formed and that ids are valid.

If we skip those checks (for the time being only) and let gumbo fix things if needed and use multiple threads to update and extract the pieces to be merged, I can cut down that merge time significantly.

But that is a bruiser of a test case once merged. It has over 40,000 lines of html code.

We synchronously wait for syntax highlighting to be done on purpose to better keep track of actual editing changes vs xhtml syntax colouring changes.

Not sure I want to change that given how rare extreme cases like this are and they should be immediately split so should be short lived issues.