I can work at about 75% capacity for days on end, and then lose an entire week if I get bogged down, or seriously focussed. Either way, the work is good, and usually enjoyable, so doing a few things to keep the brain lubricated in the off-moments doesn't cause any guilt.
And I agree with lurkette: if you're performing up to expectations, it shouldn't bother you if you finish quicker than most.
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