Electron apps are basically a Chrome window with a webpage loaded. This means that, for example, they use HTML to display the UI. HTML/CSS is complex and very dynamic, this makes it resource intensive to parse and load. Then, the whole application is written in JavaScript which is a language that is "far from the metal" (although this is getting better due to a massive investment of web browser developers).
There is also a more hidden problem, which is that Electron app developers often come from a web background, which is a field with little concern for proper resource management and performance. If a webpage has a memory leak, just refresh it. If it takes too much memory, some other inactive tabs will get killed and be refreshed. VS Code is a good example of an Electron application made by people who are concerned by performance. However, developing a performant application using Electron seems to be harder than actually doing it in another language/framework.
