Quickly checking email costs more than the minute.
Sophie Leroy showed in 2009 that switching from an unfinished task to a new one leaves part of your attention stuck on the old one; she called it attention residue. People did measurably worse on the second task when the first was left incomplete. Even finishing helped only under time pressure: only then did the brain let go.
Useful: Before switching, write one sentence on where you'll pick the old task up.
Source: doi.org