Why does interleaving feel worse than it works, and what is that illusion called?

It's a metacognitive illusion — blocked practice produces smooth in-the-moment performance that we misread as learning, while interleaving's struggle we misread as failure.

Learners consistently rate blocked practice as more effective even when interleaving demonstrably teaches them more — they judge learning by how fluent practice feels, and blocking feels fluent. This is a metacognitive error: confusing current performance with durable learning. The discomfort of interleaving is the desirable difficulty doing its job, but our monitoring system reads discomfort as "this isn't working." Trusting the evidence over the feeling is the whole discipline of studying well.