I think it's just reading unaligned. That's just a ~2x loss of throughput from L1, but the second the problem is large enough that the work being done doesn't r

I think it's just reading unaligned. That's just a ~2x loss of throughput from L1, but the second the problem is large enough that the work being done doesn't reliably fit into the L1, that doesn't matter a bit anymore. In general for x86, unaligned writes are worth doing work to avoid, but reads are in most situations not really an issue.