Having an uninterrupted and stable internet connection is probably more important than having an extremely fast connection (when in-game). My connection is slow and its download speed will be only around 2.0 Mbps – it's extremely rare I'd get any lag. The game runs super smooth. However, if my Wi-Fi signal was weak or unstable trying to play the game would be a complete nightmare. You'd be seeing lag spikes and drop-out, rubber banding, DC'ing and allsorts on a near daily basis.
It's called "bandwidth" mainly because its 'width' rather than 'speed' (think water-flow through huge wide rivers and tiny steams rather than just speed itself), also things like latency affects the game play – you don't want a high latency connection with DDO.
If you would like to read a tiny section out a book, that I co-authored (2011) covering the complex topic of: TCP/IP.
Myself said:
User Datagram Protocol (UDP)
UDP* is a connectionless "unreliable delivery" protocol operating at Transport Layer 4. As a result, UDP has no requirement for receiving protocols like TCP (also Layer 4) to acknowledge the receipt of data packets. It doesn’t concentrate on establishing connections like the TCP protocol, so it can transmit information faster than TCP; it is the upper application layers that are used to control its reliability. UDP is also useful for VoIP (Voice over Internet Protocol), streaming multimedia, and online multiplayer games moving small quantities of data—it's built for speed.
* [...] If UDP were a car it would be a lightweight Formula One racing Car. ...
In-game DDO needs to move fairly small quantities of data very fast; relying heavily on UDP, which as I explained is an "unreliable delivery" protocol primarily built for speed, etc. I feel my footnote was a good analogy meaning; it needs to move data very fast and isn't at all heavy.