The consumption sounds strange, could you give me more details? When I check "about:performance", I can see the "Energy Impact" as Low(0.35), which is higher than others but still pretty low (single Spotify tab not playing any music has Medium (6.05)).
But memory wise it should be pretty good - right now it shows only 3.2MB.
Regarding the "preview cropping mechanism", I'm opened to suggestions :). Please contact me to my support e-mail.
I'm always to receive a feedback.