I have this problem as well. I would see people who sit in meetings and send emails do very low-effort work all day, and think they were working hard. When a former manager started cracking down on developers walking outside to cool down and think or spending any idle time, I started putting the similar amount of thought and mental effort into my job, to get 8 full hours of 'work' done without burning out. During this period, I got basically nothing that wasn't super routine done. Any minor hinge in development would take days or weeks to work through instead of a few hours. Said manager was perplexed and began complaining constantly about how the pace of development was slowing down and that milestones were being missed.