I am not sure why you would doubt that running two or more instances of Chromium uses significantly more memory than one, all else being equal (i.e. the same set of loaded sites).

As for security, as a user you can be sure that a PWA is bound by the browser's security policies which are in general quite restrictive even for installed apps. An Electron app is not bound by any browser security policy and can do literally anything that a native app can do. And if the app has any kind of JavaScript injection vulnerability then those capabilities are exposed to malicious code. There's a history of Electron apps having vulnerabilities like this. Furthermore, browsers are frequently updated to address security issues in the browser engine, while Electron apps almost always receive the same updates much less promptly. So if the Electron app ever renders HTML or JS that comes from the network (which most of them do in one way or another), you are vulnerable to those issues too.

The browser updates monthly with security patches. Electron's engine lags many versions behind.

> > dramatically improving security

> How so? That seems pretty dubious to me

With an electron app, I have access to your file system and can start deleting things. Yes, some operating systems might gate access but it's not standard or granular enough.

While the file system api in browsers requires your approval.