Are you sure you have 10,000 footnotes? We don't really test Zotero with documents of that size and there may be issues, including just slow performance. There is little we can do about it, as it's a limit of the word processors.

Firstly, I have to start the footnote numbers at 1 for each chapter but Zotero does not seem to have caught up so the n numbers are incorrect.

Zotero should follow whatever Word settings you have for footnotes. If you insert footnotes manually in Word are they numbered correctly?

Secondly, when adding new references, it no longer seems to put ibid or n when the source has previously been referenced, the reference just has a dashed underline instead.

Citations have a dashed underline when automatic citation updates are disabled in the Zotero Document Preferences, indicating that they are not in their final form. When you press Refresh in the Zotero tab those citations will be updated, including ibid where necessary.

Thirdly the bibliography takes ages to appear when I press enter.

There is little to be done about this. Zotero needs time to process your document and update citations.

So for example the footnote now says Smart (n 574) even though there is no longer a footnote 574. Do you have any recommendations about how to fix this? Thanks

There is no fix -- Zotero's supra notes always refer to the full numbering of notes in the document, it doesn't/can't take into account Word settings to restart those at different number or in sections. Only thing you _can_ do is to consider keeping the chapter separately (though that can potentially cause other issues, so you need to think through that depending on your needs).