ECC is not inherently slower. Registered modules are slower (by the added latency of the register). ECC-or-not and registered-or-not are two orthogonal features

ECC is not inherently slower. Registered modules are slower (by the added latency of the register). ECC-or-not and registered-or-not are two orthogonal features of the DIMM and in theory all four combinations are possible. On the other hand non-registered ECC is somewhat rare (and for some reason often ridiculously expensive) and registered non-ECC probably is not manufactured at all (it does not make much sense).