this is a performance optimisation.
measurements using a large wallet with 11k txns:
syncing XPUB for the first time takes 10 seconds. leaving window open, and
syncing same XPUB again in new window takes 30 seconds. in third window,
it takes ~50 seconds. then ~70s. presumably scaling linearly.
this is due to the history_list.update_item call being CPU-heavy.
now all of them take 10 seconds.
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