Yes! we had this problem in year 7 with my DS at his Grammar school. we emailed his head of year who was supportive and said DS should NOT spend more that 30 minutes on each subject given. If he was, we should 'round it off' and write in his planner and sign that he had done the required amount. It was easier said than done initially but because we had the back up email from head of year we did do it a few times. Luckily, DS speed up and actually managed to do more as the terms went on.
Its such a lot to manage early on - I know!! All kids are different and progress at different rates, even at grammar and I think there needs to be a bit more give and take in this in the early weeks until they find their stride.
They all need down time in the eve especially if getting home late after traveling - dont feel bad at trying to sort this before it becomes a bigger issue.