What design differences make ram faster than ssd for read/write
The biggest difference is that SSDs are based on NAND flash cells with the serial structure shown below.



Where DRAM accesses all of the bits in a row at the same time, NAND flash serially accesses the row.  This significantly slows down read access versus DRAM.

Write access is significantly slowed because flash uses a floating gate which requires orders of magnitude more programming time than the capacitor in a DRAM cell. In most cases, since the write can only set a bit to 0 and erasing is required to set a bit to 1, writes also require erasing before programming.  DRAM writes can set either state with no speed penalty.