she takes a week to reply and it is somehow going well

Everything I have ever read here says a conversation that goes slow is dead. Keep the momentum, move it along, do not let it drift.

I have been talking to somebody since May. She replies about once a week, sometimes longer. When she does reply it is three or four paragraphs and it is obviously written by somebody paying attention, referring back to things I said a fortnight ago that I had half forgotten.

Meanwhile the fast conversations I have had in the same period have all burned out inside ten days.

Is the momentum thing just wrong, or have I got lucky with one unusual person?

The advice about momentum is written for the average case and you are not in it. What matters is whether the pace is consistent and whether the messages are worth the wait, and you have described both. A reply that took six days and clearly took twenty minutes to write is a better sign than four one line answers in an evening.

It is also worth knowing that a slow correspondence and a slow life often go together, so do not expect the pace to change once you meet. That is fine if you want that. It is a genuine mismatch if you were quietly assuming the real version would be faster.