If it's internal, you probably shouldn't be using the API Gateway. You can just call the underlying lambda directly through the SDK with invoke and save yourself the cost of the API Gateway, as well as be able to invoke it asynchronously should that be a better fit.
That said, it depends on your design/architecture as to whether that is appropriate or not. Sometimes you want that distinct separation. But if that's the case, you should also probably not even be thinking about that low latency case, because you should treat it as a service outside of your control, that may be (and probably should be) cross regionally available (so you might even be crossing the country, for all you know. This presumes the inclusion of latency based routing with a custom DNS, of course, but for a standalone, client facing service, that's what you should be doing anyway).
Obviously there may be a middle ground you want, but I'd say you probably either want to treat it as "your" API, and thus, you can be as pragmatic as you want when accessing it, or you want to treat it as a reusable, completely separate service, that you are a client of, in which case things like location can not be guaranteed, and thus latency is a reality.