Hello,
There can definitely be speed discrepancies between two devices due to architectural, hardware, and software differences. However, could you please check a few things?
First of all, make sure you have the latest firmware version installed on your device. Firmware is available here.
Secondly, ensure that software flow offloading is enabled in Network -> Firewall settings.
Additionally, it may be that due to differences in supported frequency bands and the internal modem's logic of the two devices, the TP-Link Archer uses different frequency bands. Hence, when using the TP-Link Archer router, check which frequency bands it uses and set the same bands on the RUT950 in Network -> Mobile -> General. There, you can select the Network type and set band selection to 'Manual'.
You can also find information regarding mobile and third-party antennas on our wiki pages here and here, respectively.
Kind Regards,