The algorithms are designed, at each step, to need a random memory location — the idea is you had some data, then use that data as the index into your memory.

The algorithms are designed, at each step, to need a random memory location — the idea is you had some data, then use that data as the index into your memory. This means the speed of the algorithm is measured in random memory accesses, and that speed is much slower for discs than RAM, even with SSDs.