Fast is relative here. These are microcoded instructions, which are generally terrible for latency: microcoded instructions don't get branch prediction benefits, nor OoO benefits (they lock the FE/scheduler while running). Small memcpy/moves are always latency bound, hence even if the HW supports "fast" rep store, you're better off not using them. L2 is wicked fast, and these copies are linear, so prediction will be good.

Note that for rep store to be better it must overcome the cost of the initial latency and then catch up to the 32byte vector copies, which yes generally have not-as-good-perf vs DRAM speed, but they aren't that bad either. Thus for small copies.... just don't use string store.

All this is not even considering non-temporal loads/stores; many larger copies would see better perf by not trashing the L2 cache, since the destination or source is often not inspected right after. String stores don't have a non-temporal option, so this has to be done with vectors.