It is possible, though I can’t imagine there’s much enthusiasm to rewrite the HTML generation on the off-chance that Windows Defender behaves better as a result. It would be a good thing to do from the point of view of generating better HTML, and if that helped the Defender problems that would be a bonus, but I don’t expect this to be a high priority.
A thread per-se isn’t the answer, as Inform 7 works like many compiler front-ends: there is a command line process (“ni.exe”) that does all the work, including generating the index: it is this process that Windows Defender is slowing down, since the front-end waits for it to complete. It might, in principle, be feasible to have ni.exe write the Inform 6 generated code first, then signal to the front-end that the next step could start while it generated the index. However, that requires considerable thought, and imposes an on-going cost in terms of ni.exe having to honour its new contract to always generate the Inform 6 code first. I am considering it, but don’t expect anything any time soon.