Is there a way to efficiently use Lit without using a bundler?

Lit's just a JavaScript library published as standard modules, so it doesn't require a bundler. Using Lit efficiently is the same as using any other library.

HTTP/3, import maps, and HTML preloads can make unbundled app deployment almost reasonable in some cases. You'd still miss out on minification and better compression form a bundle. Shared compression dictionaries will make this better though.