BEAM is not fast when it comes to CPU intensive workloads. "Better performance" has many dimensions, in many (most?) scenarios of web serving the bottleneck is latency to other services (i.e. Postgres) and not the processing of the web server, like you suspect in this case as well.
BEAM/Erlang/Elixir's main advantage IMHO is parallelism and "cheap" concurrency that's easy to wrap one's head around. It reuses multiple cores, and processes (lightweight threads) are both cheap (smaller stack size, per-process GC) and easy to get right (due to immutability of data structures and supervision). Also you'll get less spiky latency even under high load.
Node.js main advantages are the breadth of the library ecosystem, industry familiarity and its JIT compiler (if used correctly). At the cost of more setup one can also run an instance per core if they care for parallelism.
So I'd suggest that with BEAM he'd get quite a good performance both in terms of developer time and in serving many (network bound) requests on a multicore machine.
I can't tell if I'm touching the right points here, so just sharing a couple of thoughts:
Ideas are exciting, especially in the beginning. When you decide to work on a product, you are giving up on an endless amount of new (read: temporarily more exciting) ideas that come up to you as a creative person. This is a tough choice to be made, have you made this choice?
What does success look like to you? Is it the ability to work on what you like? If so, any kind of commitment to work on one product is going to be a difficult.
Also, you mentioned second guessing due to competition. You may notice that in any field there are many products which seem, like you said, "closely related". Thats because they only look similar from the outside. In reality they have something different about each of them, something significant enough for their makers and for the market. For example, they may be focusing on different aspect of the same problem or may be targeted at a different stakeholder or reach the same customers in a different way. Learning to find this differentiation in existing products might help you see more clearly where your product is unique.
Here's how it might look from the client side (computed style at Inspect->Style):
.elementor-875 .elementor-element.elementor-element-230321ec { text-align: left; color: black; font-family: roboto; font-size: 21px; font-weight: 400; line-height: 1.6em; }