NAT itself is a hack that causes all kinds of problems, ideally you shouldn't need it at all. Having consistent routable addresses both sides is how IP was always designed to work. NAT was originally described in RFC1631, where it's clearly described as a short term temporary hack suffering from all kinds of detrimental side effects until a proper solution can be rolled out.I am actually surprised that this is a hack, because while on my ac66u, I did not need to perform any NAT reflection configuration and it just works when accessing my nextcloud instance with ddns
The difference you're seeing is a device intended for consumers that hides the details from you and applies the most common use case vs a device aimed at people with more networking knowledge that provides more fine grained control incase you have different requirements. An externally facing service might be intentionally unreachable from the inside on some networks.
Having NAT reflection means the traffic will go from your device to the firewall, then back from the firewall to the target host instead of directly from your device to the target host. Wether you notice a difference is going to depend on the performance of the devices in question, but the fact remains is that it's an inefficient indirect route instead of an efficient direct route.
Assuming your devices are all 1gbps and capable of saturating this rate, then one client downloading direct from the server will consume 1gbps downstream on the switch while the server will consume 1gbps upstream on the switch. Every other port will be unaffected, so another client will be able to download or upload 1gbps through the firewall.
If you add reflection, then the firewall will be transferring 1gbps full duplex on the internal interface receiving data from the server and sending it to the client. If you then have another client trying to access an external site, it is going to be competing with your internal server for the 1gbps of available bandwidth the firewall has on the internal interface.