Great question — answering as the UphillTech team:
Google wants to provide its user base with the best experience possible when browsing the web - this is what retains their customers. A poor page load speed can have a serious effect on user experience, that is arguably the main reason Google sometimes ranks these sites less favorably. It is also an indication that the site isn't perhaps maintained to a high standard therefore reducing the quality of that site.
Full answer by Double Clicked: https://webmasters.stackexchange.com/a/42972 (CC BY-SA 3.0).
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