TL-WA901ND has a 100 Mbit/s Ethernet interface. So speed of data packets is limited to this. Theoretically, wireless frames could travel at max. speed if there are no interferences caused by other devices in same frequency band. But overhead in wireless communication will allow only about 50% for the interface's data speed over maximum achievable wireless speed, which could theoretically ~100 Mbps out of the 300 Mbps possible with TL-WA901ND due to the protocol overhead (e.g. wireless frames encapsulating Ethernet frames encapsulating TCP frames encapsulating IP frames encapsulating data). So, with a wireless link between two 100 Mbps data interfaces you usually don't get more speed than 50% = ~50 Mbps. Furthermore, latency can bring it down to 41 Mbps and that is what you observe.

DeltaFr_Eagle wrote

So, If I an understanding what you say correctly, I can only achieve 300mbps speed between two interfaces connected to the accesspoint ... not internet and an interfacedue to the 10/100eth port ?

Yes. The wireless speed of 300 Mbps is kind of a reserve to keep up with (nearly) 100 Mbps data coming in on the Ethernet interface in situations, where the AP gets only a fraction of AirTime to send the data over the air. Note that only one device can occupy the wireless channel at the same time to send data, no matter wether it is a client's device or another AP. The AP could need to send data over the wireless link much faster than it comes in on the Ethernet interface to keep up with the speed, because it has to release the channel often, so that connected devices can acknowledge receipt of this data.

Furthermore, b/c Ethernet is full-duplex, the maximum theoretical speed is 100 Mbps in each direction (send and receive), which yields 2x 100 Mbps = 200 Mbps. But wireless communication is only half-duplex, since at any time only one device can send.

Wo If I wanted to benefit from my 125Mbps of internet I should buy an AP with Gigabit port ? Or maybe use the AP as a repeater would be more efficient since I would not use the eth port !?

If you really get 125 Mbps out of the line, yes. But that's the same thing. If an ISP claims to deliver X Mbps data, in reality it is most often X-Y% Mbps, where Y% depends on many factors such as protocol overhead and shared use of the backbone's lines to nearby customers. This value of Y% can also vary during the day. That's the reason the ISPs most often guarantee only "up to X Mbps" for consumers, while for business-class leased lines they guarantee "at least X Mbps".

I personally would not necessarily use a Gigabit router for "up to " 125 Mbps ISP downlink speed, but I surely would do so for "up to" 200 Mbps and above.

You're welcome. Have fun with your TL-WA901, this are really fine and inexpensive devices IMHO.