> When you first posted I was intrigued, so I wrote a powershell script

I didn't post that 2015 message.

> to merge ztw.hst files. But because the script ran slower than I would
> like and I wasn't loosing config I didn't post the script. Recently I
> have suffered a number of missing history list items so I had another
> look, and then your re-post.
> The performance is still not what I would like, on my home system the
> merge takes 12 seconds.

I assume it would be pretty quick with compiled code.

Did you try doing a simple combine and sort, then eliminating consecutive duplicates? I think Powershell can do sort | unique if I haven't misunderstood the commands. You'd lose the ordering of the histories though.