No, you don't have less speed if you divide the 4-pair cable in 2 ports. If you have 10 Mbit network, you will have 10 Mbit on non-split and split cables without any difference.
The reason for splitting the 4-pair cable is simple:
As you mentioned, ethernet (10Base-T and 100Base-T) uses only 4 wires (actually, pair 2 and 3 from the 4-pair cable). That means that we can use pair 1 and pair 4 for something else. The usual procedure (although not a recommended one, the recommended solution is to use dedicated 4-pair cable for each port) is to use pair 1 for telephone.
That means that you have two outlets and pair 1 connects to the first outlet and the outlet is used for voice, pairs 2 and 3 connect to the second outlet and the outlet is used for data.
Sometimes, the cable is split like this: pairs 2 and 3 connect to the fist outlet, pairs 1 and 4 connect to the second outlet. In this case, both outlets are used for data.
Hope I answered all your questions.
Roman Kitaev, RCDD