Most of the time an "API Gateway" provides nothing. Usually this kind of service is just looking at the request path and maybe the Authorization header. In general it's just a useless hop in the TCP connection chain from the client to the server.
Somehow micro services and cloud native turned into that. So yeah, slow response times, wasted infrastructure and most work put into I/O and serialization/deserialization.
I think we forgot what actual monoliths look like and just kept decomposing… I guess that is an appropriate term.
This personal assertion only holds if you somehow chose to adopt a component that you need, or blindly adopt components even though you have no idea what you're doing. Discussing these scenarios is pointless and a waste of time though.
Meanwhile, reverse proxies and ingress controllers are fundamental components in any service that outgrows a box under the desk, and a API Gateway is nothing more than a specialized reverse proxy whigh offers some high level features as part of it's happy path.