What does latency have to do with trransfer speed ?

There is no direct relationship between latency and transfer speed. The latenycy, or RTT (round-trip time) measures how quickly a small packet can get from your computer to a server and back, however it does not measure how much data (how many packets) can be transfered in a given period of time. Two different ISPs with the same transfer speed, or even the same computer, at different times and when connecting to different servers can give you very different RTT values. Latency is related to the distance, as well as your ISP's peering arrangements, network congestion at the time, etc. however it does not relate to your available bandwidth or transfer speed.

With all that said, there is an indirect relationship between transfer speed and latency. For example, a packet takes time to travel from a server to the client, and there is a limited number of packets that can be sent (in a TCP/IP data transfer) before the server stops and awaits acknowledgement for already received packets in order to continue, so excessive latency can have a negative effect on transfer speed, especially with untweaked PCs on a broadband Internet connection.