I'm not sure if regex engine (or treesitter) + LSP really is faster than just using the LSP, as most of them check the source on typing anyways, so the overhead

I'm not sure if regex engine (or treesitter) + LSP really is faster than just using the LSP, as most of them check the source on typing anyways, so the overhead is sending the tags for the syntax highlighting. And there is no need to send the whole text, the client can send the current edited range only (if the server supports that). And at least the highlighting would actually be correct (or consistent with the compiler's AST ;).