Raw performance means nothing, it's just yet another metric that can be traded off in favor of other aspects that make up a good application. In VSCode's case, it was traded off in favor of ease of development, which spurred an extremely active and ever growing ecosystem of extensions. Was it worth it? The download counter says yes, because despite it being "slow" compared to other editors, the tradeoff is not even noticeable by most of its users.

This all boils down to the art of "it's good enough". Take game development for an example. You could write an engine from scratch using Vulkan APIs and all that jazz and run at 144fps@4k on a toaster. Or, you know, you could trade off the performance and settle for just using Unity and optimizing wherever possible. It's not as fast, but as long as the user is not frustrated by it, who cares? You just saved a lot of development time. Tradeoffs, tradeoffs.

Same thing applies here. The VSCode team did a damn good job of keeping performance just about over the "good enough" threshold of most of its users, <flamebait>unlike other Electron based applications</flamebait>. Of course, that threshold varies based on the user and his machine, but outright dismissing VSCode based solely on the assumption that editors cannot be written in html+js is simply short-sighted.