They're similar! The author of Janet previously wrote https://fennel-lang.org/, a compile-to-Lua language.

Janet has more traditional scoping rules than Lua. Tables and arrays are separate types in Janet, and arrays are 0-indexed. Biggest runtime difference is probably that Janet has value types.

I think the compile-time programming is the real differentiator, but it's hard to summarize what that means in a comment.

Performance is pretty similar to the vanilla Lua interpreter in the benchmarks I've seen and run (Janet typically wins slightly), but there's no LuaJIT.