Reading papers that turn out to be irrelevant to the specific problem at hand is probably the biggest time sink; it's also probably an important source of general education. But good academics presumably know the importance of keeping an open mind and general learning.

If someone is at the level where they need 10-20 papers to understand a topic, they are not at the level where they are even capable of asking a specific enough question. In their case, doing the hard work and sifting though 100s of papers is the best way to train themselves to think critically and thoroughly evaluate whether a paper is relevant to them.

There is also the real fact that the greatest discoveries usually come from obscure corners and reading as much as you can is the only way to explore those corners. Otherwise, you're just refining what was done before you