Tree Sitter doesn't do a tenth of what most LSP servers do, so it's much easier. But they aren't really related. Tree Sitter is a parser that you might use in an LSP server (I have done; worked decently).
Treesitter is the wrong solution anyway. The biggest advantage of LSP is that the parser, type checker,... used by the LSP is (well, can and should be) the same as the one used in the compiler (or at least another _complete_ implementation of the language, like clang for gcc/g++).
Worth noting that major languages seem to be moving away from this model. Rust and C# both concluded that the needs of your actual compiler are very different than the needs of an incremental tool that can run on partially written code.
Significantly easier because there is just a lot less surface area.