It affects both. These systems are vastly more complex than the naive mental models being discussed in these comments. For one thing, going chip-to-chip is not

It affects both. These systems are vastly more complex than the naive mental models being discussed in these comments. For one thing, going chip-to-chip is not a faultless process and does not operate at the same speed as on-chip communication. So, yes, throughput can be reduced by splitting a computation across two chips of otherwise equal speed.