Earlier quoted context omitted.
Neither you nor the poster provide a reason why. Why is it better? How does plain text make it faster to respond? Or even faster to read?
I would say that the real performance drain is top posting, which seems to go hand in hand with HTML emails. With top posting, in order to read a mail - especially one that is a few levels deep in a thread - becomes an exercise in jumping back and forth, trying to find context and make sense of who replies to what, finding relevant sentences (often hidden in a mess of signatures). With bottom posting and proper curat…
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in email?