That's normal across all industries. If there are people who are actually productive for 7 hours per day, they are extreme outliers.
I used to think that I worked 8-10 hours per day. That was until I measured my actual productivity.
I worked as a translator once. When I timed myself, I found that I can translate about 2,000 words in 1 hour. But in a day, I struggle to do more than 5,000 words, no matter how hard I push myself- even though that's just 2.5 hours of work! And if I do 5,000 words every day, I burn out within a week.