OK, now we're getting somewhere.
With an ethernet cable straight to the router, you should see ~50Mbps.
Any WiFi in the middle will almost certainly change that.
So your new TP-Link router talks to the original router via WiFi, and then cable to your PC?
The wiFi environment between the new router and the original WiFi source is probably the culprit.
If all you were getting before was 16Mbps via WiFi, and the new router is in mostly the same location, you won't get a whole lot better. It is still seeing the same overall signal.
Again, a clearer picture of your whole environment from end to end would help in diagnosing. As it is, I'm just guessing.