AX4200 RBR750 and RBS750 downloading half the speed

What Firmware version is currently loaded?
What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too?

What is the size of your home? Sq Ft?
What is the distance between the router and satellite(s)? 30 feet or more is recommended in between RBR and RBS to begin with depending upon building materials when wirelessly connected.
https://kb.netgear.com/31029/Where-should-I-place-my-Orbi-satellite

What channels are you using? Auto? Try setting manual channel 1, 6 or 11 on 2.4Ghz and 40 to 48 channel on 5Ghz.
Any Wifi Neighbors near by? If so, how many?
https://kb.netgear.com/19668/Link-Rate-and-Transfer-Speed

Try disabling the following and see:
Armor, Traffic Meter, IPv6 and Set 20/40Mhz Coexistence to 40Mhz only. Set CTS to 2347. Save settings and reboot the router and satellite(s).

One User Experience/Configuration:

https://community.netgear.com/t5/Orbi-AX/RBK753S-update-Rock-solid-in-terms-of-speed-and-stability-since/td-p/2095781/jump-to/first-unread-message