Both can fail. Failing NAND flash (solid state storage) usually results in reallocated sectors or slower reading due to retrying reads on bad sectors. Slow read/write can also just be a symptom of a nearly full drive. Failing DDR# memory can cause software freezes, crashes, or other random behaviour because usually there's no error correction at that level (ECC isn't common for consumer devices). Lots of detectable errors (through ECC) can cause a slowdown as it needs to redo the read/write.