The Importance of Attending (even boring) Talks
There are two reasons. First, 50% success rate is not bad at all. Considering enjoying a play (for me) has a probability of about 12%, and they cost money, and I still consider them worth seeing, 50% is pretty good. The reason is that the ones that are really good are so good that they make up for the lost time at the others. You might see an hour long talk that is so good for your work that it's worth having spent many hours in boring talks.
The second reason is more subtle. When you're in a boring talk, your mind wanders. This is a very good thing. Academics nowadays are so busy that they have very little unstructured time to daydream and have free-ranging creative thought. Always bring paper and a pen to a talk. When your mind checks out after the first 5 minutes, think about your big projects, your future, some problem you have not figured out, your big theory, your time management, what your priorities are, and start writing. Make a list of things to put on your todo list. I go to a lot of conferences, and my notes are legion at the end of it, and many of the notes are not really about the talks at all. The talk might inspire some idea that is only tangentially related, and spur a whole new creative project. When I get back, and go over the notes, I'm always surprised at how many ideas I had. I'm also always shocked at how many I know I would have never remembered had I not written them down.
Being bored for a time can be valuable. The boredom forces your mind to find it's own entertainment, and often you dig up problems you are too buy to think about any other time. Embrace the boredom.