They're not preloading your emails, just making it look like it, by preloading a few fields (from, subject). Providers have to pay for every byte they send, so they make damn sure they're not sending a bunch of extraneous data, at scale, for millions of users.
Specifically for "old emails" in my mind, these are emails you'd have to dig for. The act of digging up the old emails will be much more data intensive than leaving them sit, which draws no power. Your old emails aren't kept in RAM.