I am using a RUT950 with LAN devices and Internet on WAN port with 100MBit/s. The throughput is max 45MBit/s while when connecting e.g. a laptop directly to the WAN device without RUT it is increasing to 97MBit/s, so almost 100. I use only a very few FW (default) rules and they are not complex, no NAT. The rules not used or not required I deleted - to be on the safe side.
Betwen RUT and end user devices is a simple 8 port switch.
Could you try closing the WebUI and disabling the unnecessary services on the device when running a speed test? RUT240 is quite processing power limited, thus it’s worth checking whether performing these actions will help.