Oh, man, how can you be wrong about everything? Here is the author noticing performance issues, he's even in a "but in theory it doesn't have to be slow" camp that's closer to yours:

> An app can use a lot of memory, and it does not necessarily mean it’s a performance nightmare, but the issue with the new WhatsApp is that it feels sluggish. You’re going to notice sluggish performance, long loading time, and other performance issues when browsing different conversations.