Question: maximum sound output in dB provided by 3.5mm Audio port in Raspberry pi3?

Answer: 
The reason it uses 10Mbps is because whatever it is connected to only supports 10Mbps.

```
Link partner advertised link modes:  10baseT/Half

```

A Gigabit router would say something like:-

```
Link partner advertised link modes:  10baseT/Half 10baseT/Full 
                                     100baseT/Half 100baseT/Full 
                                     1000baseT/Full 
Link partner advertised pause frame use: Symmetric
Link partner advertised auto-negotiation: Yes
Link partner advertised FEC modes: Not reported

```
According to the [specs](https://www.raspberrypi.org/products/raspberry-pi-3-model-b-plus/) your model (3B+) will not run at full gigabit ethernet speed due to the limitations of the USB bus:

> 
> Gigabit Ethernet over USB 2.0 (maximum throughput 300 Mbps)
> 
> 
> 

Consider upgrading to a RPI 4 if speed is important to you.

But what about your router and switch ?
Any device standing between your ISP modem and your Rpi can potentially be slowing you down. 100 Mb ethernet interfaces are still common on domestic network appliances.

Ethernet cables ? You should have at least CAT5e/CAT6 cables to take advantage of that speed.

Try to make a direct connection from the Rpi to your Internet modem (or FTTH or whatever) with a quality ethernet cable, and measure performance again.