I changed a few very simple Lambda routines running Node.js v14 from "x86_64" to "arm64". Not complicated modules that run relatively quickly doing some very basic calculations. I was seeing double the running time. One routine that was reporting a billed duration of 2049 ms with "x86_64" reported 4243 ms with "arm64". Switched it back to "x86_64" and the runtime halved as expected.

So at least with Node.js, I see no benefit with the arm64 architecture. YMMV.