Having heard not so good things about Hubdoc intially I thought I would give it another go yesterday.
Firstly I used on a Xero client with over 43,000 contacts, according to Xero support it is most probably this number of contacts in Xero that made Hubdoc so slow it was unusable. According to them anything over 1000 and Hubdoc will struggle.
Secondly, I then tried with a client with 700 contacts and it worked well.
Given that over time the number of contacts in Xero is only going to increase for any client and you cannot delete a contact in Xero this seems like a serious flaw to me.