Well, not exactly. Reading this on my kindle, I can say definitively that only 6% of the book dealt strictly with gender stereotypes in math and science and its affect on women. This was exactly one chapter. The following chapter touched on the subject as well, but it wasn't the main focus, and the author was bringing all of the points she made in the previous chapters to a close, tying them all together in regards to performance pressures in academia.
Beilock is far from unbiased on the issue of gender stereotypes, so I'm not surprised it may have struck a cord with you and may do so with other readers. Also, I was less than happy that she didn't take the idea of gender biases and do a better job of painting a larger picture for what this could mean for students outside of math & science and also beyond its implications for just women (she actually makes a half-hearted attempt to sum up gender biases affecting young boys, but ends it shortly with the summation that they don't exist in math & science so, oh well, no need talking about that. disappointed.). Having said that, it didn't amount to much more than me thinking "well, she should have said this, and this, and this, and this, oh and this too... why didn't she include those things?", and this was again, no more than 10% of the entire book. Barely even worth mentioning.
As for the 100 page tangent, I don't see that either. There is a really logical structure and order to the book. She chunks main ideas into sections, giving them descriptive headings, she summarizes at the end of each chapter and at the end of each main section (failure to perform in either academia, sports or in the business world), coming back to tie all of the ideas presented in the chapters dealing with each topic together, then goes on to give a list of the techniques discussed to deal with performance pressures. I don't see how you'd call any of this a "tangent". I think maybe your experience was colored by the difficulty you had with the sections on gender biases.
This is definitely a great read. I felt like a lot of it was echoing things I've personally come to know intuitively (indeed, she talks about professionals knowing certain things intuitively that are later confirmed by research), and it was great to see that what I believed wasn't just that and was (or is) in fact a fact(s). This book is definitely edutainment, so if those are the types you like to read, you'll be glad you picked this one up.