How to attend conferences efficiently?
I guess I'd think about it a little differently than you did. It doesn't sound to me like your experience was so bad! I think it might be helpful just to reframe your goals.


It's usually true that you won't get everything from the talks. Otherwise your classes would consist of lecture and no tests or reading! Instead of aiming to follow 100%, I focus on: getting exposure to new/recent/cutting-edge developments in my field; taking advantage of the fact that verbal explanations are often high-level and simplified, and include informal "practitioner knowledge" that you won't find in papers; and finding pointers to research papers or resources that I can explore later on my own time.
I don't think it's worth trying to have a rich "travel experience" at the same time (aside from taking advantage of any social excursions organized by the conference). If you want to do tourist activities, I think a typical option is just to extend your stay a few days before/after so you can explore without the pressure of a schedule.
For me, networking is the true value of such events. The reason why summer schools exist is so that you can meet other scholars in your field (otherwise everyone could just stay home and watch online lectures or read papers). So, if you are good at networking and enjoy it, that's great! I think you did what you were supposed to be doing. It is a priviledge to be able to discuss your ideas and hear about other people's work.


Strategically, I guess the only two points I would add are:


Be discriminating about what you attend. If you didn't enjoy the experience, it's possible that the school did not truly touch on your interests, or was not a good match for your background experience (i.e. too hard/too easy). As you note, the time cost of attending is high so weigh that against "fit" of the school.
You don't have to go to every session. In my experience, many people take short breaks from the talks when the session does not match their interests, or if they know the topic already.


In conclusion, I would not recommend a focus on "being productive"; instead, I try to focus on being present at the event, meeting new people, and learning from discussions with them.