Not always, but sometimes Apple Mail uses an obscene amount of memory.
How it even possible for a competently written program whose task is to display a list of messages and little else gobble up 3.12GB of real memory? Or to take 20 seconds to show me my Sent box (all local messages). Or to rainbow beach ball for 3-4 seconds for any search, which makes it hard to even type in a short search phrase.
The answer of course is that Apple Mail engineers are incompetent. Every day my usage of Apple Mail is degraded by the bugs I discussed two macOS update releases ago. The VIP features has been broken for two major OS releases. Apple Mail is swiss-cheesed with bugs, and the severe performance problems and this excessive memory usage are just symptoms of the metastasizing Apple Core Rot (software and hardware) disease.