Networking optimization

Hello,

Here is my configuration:
The modem-router is through ethernetlink attached to the TC.
An Airport-express extends the network. (extend option under wireless).
Modem-router is disabled as my TC is also a router.

The Imac next to the TC, communicates wirelessly with the TC.

Issues:
Trouble playing back from audio-files stored on the TC. Especially from my macbook that is closed to the airport-express. Also internet-speed is not that good, even after an upgrade to vdsl-II are something like that. 10mbp/s.
All my TCP/IP addresses are DHCP. Is this Ok ? Don't have any experience with this.
My internet connection is in bridge-mode ? Ok ? Is this the best option. ?
I selected manually the TCP/IP addresses. Ok ?
Radio mode is at 2.4 ghz, 802.11n.

I expected playback from TC wirelessly to work without interrupts. But with the Macbook in the evening this is not the case.

Ideas for optimisation ?

Do you have a cordless phone or any other possible RF interference?

Do you have a long enough ethernet cable to go to the iMac?

If you turn off Time Machine on the end points does the streaming improve?

By coincidence I attached an extra ethernet cable directly from the TC to the Imac, so that the Imac is no longer wireless. So I go on the internet with the internet connection, and the airport shut off.
Playbacks of course go unhampered now, and the Internet is also far better.

At first glance, the Macbook downstairs also improved its playing back capabilities, and the Internet.
All to do with an extra ethernet link. Perhaps. Anyway, It's better now.
I still need more tests with the macbook, as yesterday I was closer to the airport-express, not much, but still, sometimes it makes a difference. And My position was also away from, how do they call that, the chimney (let's say the passage through the house where the smoke leaves the house), which gave problems before (metal housing), reason why I bought the airport-express to extend the network.

I'll see what heapens next. For the time being, it is pretty much ok.
But If my settings are ok, then I think there is not much more I can do.

Bye.