At $dayjob I work on a very dense UI for a financial institution. Think Bloomberg terminal in React and RTK. It's extremely customizable and had lots of elements updating multiple times per second (charts, graphs, enormous tables, etc.) with realtime data coming through over a websocket. We do very little actual performance tuning and mostly just follow best practices.
I don't think React is to blame for your performance issues.
If seeing how common `eslint-disable-next-line react-hooks/exhaustive-deps` is any indication, people just don't bother with best practices. Not to mention the majority of people I interview or hire don't know anything about CSS performance, they learn on the job. They'll happily add 1000 box shadows in a view if it's in the design.
Also OP is looking to move an existing React UI to Svelte, why not try Preact first instead of an entire rewrite? Or even Inferno or million.dev? If they did and they're insufficient, I don't even believe the browser's DOM is the right technology for that UI then.
I don't think React is to blame for your performance issues.