I used to be a lecturer at ANOther RG uni, and my daughter has just completed her degree at Oxford. The difference in how the student workload is structured was interesting.
At Oxford, you have a 1:1 or small group tutorial every week, and have to complete an essay for every one. In addition to the public exams, most people also get internal college exams which don't count towards your degree, but are designed for your college tutor to find out how you are getting on. The result of this structure is that you are always very conscious of needing to keep up with your work - it's really hard to cruise along under the radar at Oxford. I suspect there is also a selection effect - people who get to Oxford tend to be people who are used to working hard for tests, exams etc and often feel they have to "justify" being there - "imposter syndrome" is real thing for many Oxford students. I think the higher proportion of Firsts at Oxford (over 30% compared to around 20% UK average I believe) reflects this.
Conversely, at the uni that employed me, the workload was less constant, with some periods of slack and others of great intensity. Students who wanted to do well sometimes had to be more proactive in taking responsibility for their learning.
Overall, I do think the average Oxford student works longer/harder than the average non-Oxbridge.
But having said that, I knew many students at my lecturing uni who worked equally hard and who were equally able. And it is possible to get a First and still have a great social life wherever you go.