Do you have QoS running on your TP-Link? that can lower your throughput, I wouldn't usually expect by that much but I do not know your router, and sometimes there are just bugs with some builds of OSS firmware that have given really low bandwidth for some people until they reported it and the firmware was fixed.
If QoS is off, check for newer firmware, and also try a factory reset, especially if you did not do that when you first swapped to OpenWRT.
Do you have QoS running on your TP-Link? that can lower your throughput, I wouldn't usually expect by that much but I do not know your router, and sometimes there are just bugs with some builds of OSS firmware that have given really low bandwidth for some people until they reported it and the firmware was fixed.
If QoS is off, check for newer firmware, and also try a factory reset, especially if you did not do that when you first swapped to OpenWRT.