Turtle Request
A turtle request (or turtle transfer) is a type of a request.
If node A requests a block of data from node B, and B has the data and begins sending it, but the transfer is very slow, node A classifies the request as a turtle request.
Slow requests are handled specially in order to enable node A to respond to the node that requested the data from A before a timeout occurs.
In the case of a normal (fast) transfer, A would simply send the data onward. For a turtle transfer, that would cause a timeout, which would cause A to be treated as a slow node.
Instead, when a transfer appears to be slow, A returns a DNF upstream rapidly, but keeps transferring the data from B. When the transfer eventually finishes, A can send it onward using the ULPR mechanism.