Why do computers take more time in prime factorization than in multiplication? The time difference is too much.

How long would it take you to mix a cup of sugar and a cup of salt?

How long would it take you to separate them back?

Many actions, even reversible ones, require very different resources to do and to undo. Multiplication is very much like fusing things together: we take the digits of the given numbers and perform a straightforward but intricate dance of shifting and mixing and adding, and the result is a tangled jumble of the original ingredients. Separating it back is, as far as we know and expect, as hard as unmixing flour and powdered sugar, particle by fine particle.