Another round of speculative-execution vulnerabilities

If I were to send you a good old-fashioned letter, it would take a day or 4 (or somewhat less or more, I don't actually know; the exact value doesn't matter for this discussion); a reply from you to me would take 4 days too; there and back is then 8 days. Does that mean that I can only send you a letter once every 8 days? Only when my letter needs information from your reply; in that case I have to wait until I get your letter. But in all other cases, I can easily send new letters while old ones are still in transit. The distance between you and me sets a lower limit on latency, but does not affect bandwidth. It's the same for communication in computer systems.