Uff.... I just stumbled in here by chance and have to clarify a few issues here for the following ones:

Yes, every transmission (line, wireless) has losses over increasing distance. However, these do not occur in the dimensions that are answered here.
Cat6, for example, is defined stable at 55m with 10Gb per second. Whether you have 1m or 25m doesn't matter. According to the calculation of user Valimaus, after 25m cable you still have 7% of the performance - which is not true.
Cengiz01: 1m, 1.5m or 2m are absolutely irrelevant, especially for a TV.

Aurosis: if you're still not sure, then buy a shielded Cat6 on 25m and you'll get (if you don't wrap it with power cables like a roulade) also out at the end what was pushed in at the beginning.

Whew, some of the answers are hair-raising... Well, there are different Ethernet standards. Common are 100MBit/s or 1GBit/s. Depending on which devices IF you connect to each other. The devices say to each other hello, I can do 1Gbit, hello I can also do 1Gbit, so let's do 1GBit/s. If you have a bad cable in between, this will generate errors on the line. A good 1m cable is just as fast as a good 30m cable, apart from the physical speed of light, which you don't notice at this distance.

Dear Aurosis, the shorter the cable the faster the values arrive at their destination, and this also means a faster speed, it really makes a difference if you can save on cable length, for example if your TV is about 1 metre away from the socket I wouldn't buy an internet cable where 2 metres is, thanks rather 1 metre cable or 1.5 metre cable :)

No, the transmission rate drops by approx. 10% per additional metre (power loss).