Hi,
I have 100Mbps internet and I just noticed that I only get 30Mbps when I connect my PC to my router. I get my whole 100Mbps when I connect from modem to my PC.
I tried to restore factory setting but still no luck.
My router is D-Link DIR-619L.
To be sure, start with a factory reset again. Then set just the admin password and the WIFI name and password. Nothing else.
There are many things that you can set that will slow performance, you might not even realize it.
That is also a pretty old low-end router. You might consider an upgrade.