I have seen numbers like 30 seconds per mile slower for every 5 degrees over 60 (maybe it's 10 degrees). BUT . . . I have never seen that much loss in practice. And as shipo said, you can acclimate to the increased temps and regain some (but probably not all) of the lost performance. Honestly, it's not worth worrying too much about it, because when the cooler weather returns, you will easily regain your pace.
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