The actual RAM chips on a ECC DIMM are exactly the same as a non-ECC DIMM, there's just an extra 1/2/4 chips to extend to 72 bit words.

The main reason ECC RAM is slower is because it's not (by default) overclocked to the point of stability - the JEDEC standard speeds are used.

The other much smaller factors are:

* The tREFi parameter (refresh interval) is usually double the frequency on ECC RAM, so that it handles high-temperature operation. * Register chip buffers the command/address/control/clock signals, adding a clock of latency the every command (<1ns, much smaller than the typical memory latency you'd measure from the memory controller) * ECC calculation (AMD states 2 UMC cycles, <1ns).